Causes and Effects of 20th-Century Wars

How to Use This Guide

• Paper 2 World History Topic 11 covers three broad themes: the causes of wars, the practices (conduct) of wars, and the effects and results of wars
• Paper 2 is a 90-minute exam in which you write two essays, each worth 15 marks — drawn from different World History topics or both from Topic 11
• This topic is identical for SL and HL students — both sit the same Paper 2 questions. HL students also sit Paper 3 (regional option), but that is separate
• The single most important rule: questions almost always ask you to refer to examples from more than one region. Two Western European wars (e.g. WWI and WWII) are acceptable as contrasts, but pairing WWI with the Chinese Civil War demonstrates greater geographical range and often scores better
• Exam Alerts flag the traps that cost marks in essays
• IB Tips highlight what examiners reward in extended responses
• Worked Examples provide model essay outlines for common question types

Aligned to IB History SL/HL World History Topic 11 — current syllabus

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Videos on this page: Overview — Causes of 20th-Century Wars · World War I Causes · World War II Causes · Practices of War · Effects and Results

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Watch: Overview — Causes and Effects of 20th-Century Wars

TED-Ed · 6 min · What caused WWI? — The long-term, short-term, and immediate triggers of the Great War explained clearly

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Section 1: Theme 1 — Causes of Wars

1.1 The Framework: Long-Term, Short-Term, and Immediate Causes

The IB syllabus requires you to analyse causes across three time horizons. Examiners expect you to distinguish between these categories and to explain how they interacted.

Category	Timeframe	Function
Long-term causes	Decades before the war	Create the structural conditions; make war possible
Short-term causes	Months to a few years before	Generate specific tensions; make war likely
Immediate causes	Days to weeks before	Trigger the outbreak; the “spark”

The IB also identifies four types of causes that cut across the time categories:

• Economic — resource competition, trade rivalry, economic imperialism, industrialisation creating military-industrial competition
• Ideological — nationalism, militarism, imperialism, fascism, communism, pan-Slavism
• Political — alliance systems, failures of diplomacy, domestic political pressures, leadership miscalculation
• Territorial — competition for land, colonies, strategic positions, border disputes

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1.2 Causes of World War I (1914–1918)

Long-Term Causes

Militarism:

European great powers had engaged in a sustained arms race since the 1870s. Germany’s naval expansion (the Tirpitz Plan, from 1898) directly threatened British naval supremacy, producing the Anglo-German naval rivalry. By 1914 Germany had the second-largest navy in the world. Military spending across Europe doubled between 1870 and 1914. General staffs developed rigid war plans (Germany’s Schlieffen Plan, France’s Plan XVII) that required immediate mobilisation at the first sign of conflict — leaving almost no room for diplomatic delay once the crisis began.

Alliance Systems:

Europe was divided into two armed camps:

Alliance	Members	Original purpose
Triple Alliance (1882)	Germany, Austria-Hungary, Italy	Bismarck’s defensive system against France and Russia
Triple Entente (1907)	Britain, France, Russia	Balance Germany’s growing power

The alliance system meant that a local conflict (Austria-Hungary vs. Serbia) could rapidly pull in all major powers. What might have remained a regional Balkan dispute became a continental — then world — war within six weeks of the assassination of Franz Ferdinand.

Imperialism:

Competition for colonies generated repeated international crises: the First Moroccan Crisis (1905), the Second Moroccan Crisis (1911), and the Balkan Wars (1912–1913). Each crisis was resolved diplomatically, but each left underlying tensions unresolved and military plans more advanced. Imperial competition intensified economic rivalries and fed nationalist sentiment in all major powers.

Nationalism:

Pan-Slavism threatened the multi-ethnic Austro-Hungarian Empire by inspiring Serbian ambitions to unite the south Slavic peoples. Pan-German nationalism fed the desire for German dominance in Central Europe. Within Germany, domestic political pressures from nationalists made diplomatic compromise politically costly for the German government.

Short-Term Causes

The Balkans — “powder keg of Europe”:

Austria-Hungary’s annexation of Bosnia-Herzegovina in 1908 humiliated Russia and inflamed Serbian nationalism. The Balkan Wars of 1912–1913 expanded Serbia’s territory significantly, alarming Austria-Hungary. By 1914, Austria-Hungary had concluded that Serbia must be crushed before it became powerful enough to threaten the empire’s cohesion. This produced the willingness to use the Sarajevo assassination as a pretext for war rather than a reason for diplomacy.

The July Crisis:

The assassination of Archduke Franz Ferdinand (June 28, 1914) triggered a chain of ultimatums, mobilisations, and declarations of war that lasted 37 days. Key decision points:

Date	Event
Jun 28, 1914	Franz Ferdinand assassinated in Sarajevo by Gavrilo Princip (Black Hand connection)
Jul 5–6	Germany issues the “blank cheque” — unconditional support for Austria-Hungary
Jul 23	Austria-Hungary delivers ultimatum to Serbia (deliberately unacceptable terms)
Jul 28	Austria-Hungary declares war on Serbia
Jul 30	Russia begins general mobilisation
Aug 1	Germany declares war on Russia; France mobilises
Aug 3	Germany declares war on France; Schlieffen Plan activated
Aug 4	Germany invades Belgium; Britain declares war on Germany

Historiographical Debate on WWI Causes

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1.3 Causes of World War II (1939–1945)

Long-Term Causes

The Treaty of Versailles (1919) and its consequences:

The peace settlement produced resentments that Hitler could exploit. Germany lost 13% of its territory, 10% of its population, all overseas colonies, and its entire merchant fleet. Article 231 (the “war guilt” clause) assigned sole responsibility for the war to Germany and its allies, justifying reparations payments eventually set at 132 billion gold marks. Military restrictions left Germany with an army of only 100,000 men. The settlement was widely perceived as a “Carthaginian peace” — vindictive rather than constructive.

Critically, the settlement satisfied no major power: France believed it was too lenient; Germany believed it was humiliating; Britain privately felt guilty about its harshness; the USA rejected it entirely by withdrawing from the League of Nations and Versailles.

The Great Depression:

The global economic collapse from 1929 destroyed democratic governments across Europe. In Germany, unemployment reached 6.1 million by January 1933; democratic parties discredited themselves through coalition paralysis. The Depression produced the political conditions in which Hitler could win mass electoral support and be appointed Chancellor (January 30, 1933). Without the Depression, it is very unlikely that the Nazi Party would have achieved sufficient electoral strength to pressure Hindenburg into the chancellorship appointment.

The Failure of Collective Security:

The League of Nations failed to enforce collective security in Manchuria (1931), Abyssinia (1935–1936), and the Rhineland (1936). Each failure taught Hitler — and other aggressors — that the international system lacked the will to enforce its rules. The policy of appeasement adopted by Britain and France reflected genuine fears of another war, but its effect was to validate aggression and encourage further demands.

Short-Term Causes

Hitler’s Ideology and Foreign Policy Programme:

Hitler’s goals were stated clearly in Mein Kampf (1925) and repeated consistently in speeches:

1. Revision of the Treaty of Versailles
2. Anschluss (union with Austria)
3. Lebensraum — eastward territorial expansion to create living space for the German people
4. Destruction of the Soviet Union and ideological war against “Judeo-Bolshevism”

The progression from the Rhineland remilitarisation (1936) to the Anschluss (March 1938) to the annexation of the Sudetenland (September 1938, Munich Agreement) to the invasion of Poland (September 1939) followed this ideological logic precisely.

The Failure of Appeasement:

The Munich Agreement (September 29, 1938) gave Hitler the Sudetenland in exchange for a promise that it was his “last territorial demand.” Neville Chamberlain returned to Britain announcing “peace for our time.” Six months later, Germany occupied the rest of Czechoslovakia (March 1939) — breaking the Munich commitment and destroying the credibility of appeasement. Only then did Britain and France issue a guarantee to Poland.

The Nazi-Soviet Pact (August 23, 1939):

Hitler secured his eastern flank by signing a non-aggression pact with Stalin. The pact’s secret protocol divided Eastern Europe into German and Soviet spheres of influence. It made the invasion of Poland possible without the risk of a two-front war that had destroyed Germany in WWI. When Germany invaded Poland on September 1, 1939, Britain and France declared war on September 3 — beginning World War II.

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1.4 Causes of the Spanish Civil War (1936–1939)

The Spanish Civil War is an excellent supporting case study because its causes combine domestic political polarisation, economic inequality, military conspiracy, and foreign intervention into a single conflict.

Long-term causes:

• Deep structural inequalities: large landowners (latifundistas) dominated the south; the Catholic Church owned vast wealth; industrial workers in Catalonia and the Basque Country organised militantly
• A weak parliamentary republic (the Second Republic, proclaimed April 1931) struggled to govern a deeply divided society
• Regional tensions: Catalonia and the Basque Country demanded autonomy; conservative centralists opposed it
• Failed land reform: the Republic’s attempts to redistribute agricultural land alienated landowners without satisfying landless labourers

Short-term causes:

• The February 1936 election produced a narrow Popular Front victory (leftist coalition of republicans, socialists, and communists). The right refused to accept the result
• Political violence escalated sharply throughout spring 1936: assassinations of left and right-wing figures, church burnings, land seizures
• Military conspirators (including General Francisco Franco) planned a coup as early as April 1936

Immediate cause:

• The military uprising of July 17–18, 1936 began in Spanish Morocco and spread to garrisons across Spain
• The coup’s failure to win immediate control of Madrid and Barcelona converted a military pronunciamiento into a prolonged civil war

Foreign intervention as a decisive factor:

The civil war became an international conflict. Germany and Italy provided crucial military support to Franco (Condor Legion, Italian ground troops, arms); the USSR supplied the Republic with arms, advisers, and political influence through the Communist Party; the International Brigades brought approximately 35,000 foreign volunteers to fight for the Republic; Britain and France adopted a policy of “non-intervention” that effectively helped Franco by preventing the Republic from receiving Western arms.

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1.5 Causes of the Chinese Civil War (1927–1949)

The Chinese Civil War between the Nationalist Party (KMT/Guomindang, led by Chiang Kai-shek) and the Chinese Communist Party (CCP, led by Mao Zedong) provides the key Asian case study for Topic 11.

Long-term causes:

• The collapse of imperial China (1912) left a power vacuum exploited by regional warlords; no stable national government existed
• The “Century of Humiliation” — foreign exploitation of China through unequal treaties, the concession system, and the Twenty-One Demands (1915) — produced intense nationalism
• Deep rural poverty: approximately 4% of the population owned 50% of agricultural land; most peasants lived as tenant farmers paying ruinous rents

Short-term causes:

• The KMT-CCP United Front (1924–1927) broke down violently when Chiang Kai-shek launched the Shanghai Massacre (April 12, 1927), killing thousands of communist activists. This converted an uneasy political alliance into open civil war
• The Japanese invasion of Manchuria (1931) and the full-scale Second Sino-Japanese War (1937–1945) forced the two sides into a Second United Front — but their underlying competition never ceased

Why the CCP won (1945–1949):

• Mao’s strategy of building a rural peasant base (rather than relying on the urban proletariat as Soviet doctrine suggested) proved decisive in a predominantly agricultural society
• The Long March (1934–1935) — a 9,000-kilometre tactical retreat — created a legendary military leadership core and embedded the CCP’s presence in northwest China
• KMT corruption and economic mismanagement (hyperinflation reached 1,000% in 1948) destroyed its popular support
• CCP land reform policies won genuine peasant loyalty by redistributing land from landlords to poor farmers
• The People’s Liberation Army’s military campaigns from 1947 quickly overran KMT forces; Chiang fled to Taiwan in 1949

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Section 2: Theme 2 — Practices of War

2.1 Types of War and Technology

The IB syllabus distinguishes between different types of war and examines how technology transformed warfare in the 20th century.

Types of war present in 20th-century conflicts:

Type	Definition	Example
Total war	Entire society and economy mobilised for the war effort; distinction between combatants and civilians eroded	WWI, WWII
Civil war	Armed conflict within a single state between rival political or social factions	Spanish Civil War, Chinese Civil War
Guerrilla war	Irregular warfare by small mobile groups against a larger conventional force	Chinese Civil War (CCP against KMT), Spanish Republican resistance
War of attrition	Strategy of wearing down the enemy through sustained casualties and resource exhaustion rather than decisive battles	WWI Western Front

World War I: The First Modern Total War

WWI introduced or massively expanded multiple technologies that transformed how wars were fought:

Technology:

Technology	Effect
Machine gun	Made massed infantry charges catastrophically costly; contributed to trench stalemate
Artillery	Accounted for approximately 58% of all WWI casualties; required industrial-scale shell production
Poison gas	Chlorine (first used at Ypres, April 1915), then phosgene and mustard gas; caused 1.2 million casualties; deeply demoralised troops
Tanks	Introduced at the Somme (September 1916); overcome trench deadlock by 1918 (Battle of Amiens, August 1918)
Aircraft	Reconnaissance, then fighter and bomber roles; strategic bombing introduced
Submarine warfare	Germany’s unrestricted submarine warfare sank millions of tonnes of Allied shipping; brought the USA into the war (1917)

The Western Front and trench warfare:

The Schlieffen Plan’s failure at the Marne (September 1914) produced a continuous front of trenches stretching from the English Channel to Switzerland — approximately 700 kilometres. The combination of machine guns, artillery, and barbed wire made offensive operations almost impossibly costly. The Battle of the Somme (July–November 1916): the British army suffered 57,470 casualties on the first day alone (July 1); total Allied casualties approximately 620,000; German casualties approximately 450,000; total territorial gain approximately 10 kilometres.

World War II: Mobile Warfare, Air Power, and Genocide

WWII reversed many of WWI’s tactical features. The combination of tanks, aircraft, motorised infantry, and radio communication (Blitzkrieg — “lightning war”) restored mobility to warfare.

Blitzkrieg:

Germany’s invasion of Poland (September 1939) and France (May 1940) demonstrated the effectiveness of combined-arms warfare: concentrated armour punched through defensive lines, bypassed strong points, and destroyed the enemy’s communications and command before a coherent defence could be organised. France — considered militarily powerful — fell in six weeks (May–June 1940).

Air power:

Strategic bombing became a major instrument of war. The RAF’s Bomber Command and the US Army Air Forces conducted sustained strategic bombing of German cities and industry (1942–1945). The firebombing of Dresden (February 1945) killed an estimated 22,700 people in two nights. The US atomic bombings of Hiroshima (August 6, 1945) and Nagasaki (August 9, 1945) killed an estimated 110,000–210,000 people, effectively ending the Pacific war.

The Holocaust as an extreme practice of war:

The systematic murder of approximately 6 million Jews — along with Roma, disabled people, political opponents, and Soviet prisoners of war — was carried out under the cover of war. The Nazi genocide demonstrates that the practice of 20th-century wars extended far beyond the military front to include the deliberate mass murder of civilian populations defined as enemies.

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2.2 The Role of Civilians

The distinction between combatant and civilian, central to international law since the 19th century, was eroded progressively across 20th-century conflicts.

Total mobilisation:

In both world wars, civilian populations were essential to the war effort. Women entered industrial employment on a massive scale (over 1 million women working in British munitions factories by 1917; US female employment in war industries doubled 1941–1944). Rationing systems, propaganda ministries, and conscription systems integrated civilian life into the war machine.

Civilian casualties:

WWI killed approximately 7 million civilians (primarily through disease, famine, and the collapse of the Russian, Austro-Hungarian, and Ottoman empires). WWII killed an estimated 38–55 million civilians — more than the military dead (approximately 22 million soldiers). The deliberate targeting of civilian populations through strategic bombing, the Holocaust, and the Japanese occupation of China (including the Nanjing Massacre, 1937) made civilian death a central feature, not a byproduct, of the conflict.

The Spanish Civil War — civilian targeting:

The bombing of Guernica (April 26, 1937) by the German Condor Legion and Italian Aviazione Legionaria was the first sustained aerial bombardment of a civilian town. It killed an estimated 150–1,600 civilians (estimates vary widely) and became a symbol — immortalised in Picasso’s painting — of the new willingness to use air power against non-combatants.

The Chinese Civil War — civilian suffering:

The KMT government’s desperate economic measures (printing money to fund the war) produced hyperinflation that wiped out the savings of the urban middle class. CCP land reform in liberated areas, while winning peasant support, was accompanied by the violent elimination of landlords. Civilian suffering was inseparable from the conflict’s political dimension.

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2.3 Resistance Movements

Resistance to occupation was a significant practice of 20th-century wars, particularly in WWII.

In occupied Europe:

• French Resistance (the “Maquis”): carried out sabotage, intelligence gathering, and the concealment of Allied airmen; the Normandy invasion (June 1944) depended partly on Resistance disruption of German communications
• Polish resistance: the Home Army (Armia Krajowa) was the largest European underground army; the Warsaw Uprising (August–October 1944) saw 250,000 civilian deaths when the SS suppressed the rebellion
• Yugoslav Partisans: Tito’s communist-led Partisans tied down approximately 35 German and Italian divisions — a significant diversion of Axis resources

Resistance in China:

Both the KMT and the CCP developed extensive guerrilla resistance to Japanese occupation. Mao’s writings on guerrilla warfare theory (notably “On Guerrilla Warfare,” 1937) provided a systematic framework — gaining popular support, establishing base areas, and conducting hit-and-run operations — that influenced resistance movements globally.

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Section 3: Theme 3 — Effects and Results of Wars

3.1 Framework: Categories of Effects

The IB syllabus organises the effects of wars into five categories. Examiners expect essays to address at least two or three of these to demonstrate analytical range.

Category	Key questions
Peace treaties and settlements	Did the peace create lasting stability? Were the terms just?
Political effects	How did the war change governments, borders, and political systems?
Economic effects	Who paid for the war, and how? What were the long-term economic consequences?
Social effects	How did the war change gender roles, class structures, and social norms?
Demographic effects	How many died? What were the population consequences — displacement, migration, ethnic changes?

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3.2 Effects of World War I

The Peace Settlements (1919–1923)

The Paris Peace Conference produced five separate treaties:

Treaty	Defeated power	Key terms
Versailles (Jun 1919)	Germany	Lost 13% territory, 10% population, all colonies; reparations (132bn gold marks); 100,000-man army; Article 231 war guilt clause
Saint-Germain (Sep 1919)	Austria	Broke up Austria-Hungary; created Austria, Czechoslovakia, Yugoslavia; Anschluss with Germany forbidden
Trianon (Jun 1920)	Hungary	Hungary lost 72% of its pre-war territory; 3.3 million ethnic Hungarians left in neighbouring states
Neuilly (Nov 1919)	Bulgaria	Lost territory to Greece, Romania, Yugoslavia
Sevres (Aug 1920) / Lausanne (1923)	Ottoman Empire	Dismembered; Turkish War of Independence reversed some terms; final settlement created modern Turkey

Political Effects

• Collapse of empires: Four empires — the German, Austro-Hungarian, Russian, and Ottoman — ceased to exist. New states emerged: Poland, Czechoslovakia, Yugoslavia, Finland, Estonia, Latvia, Lithuania
• Rise of communism: The Bolshevik Revolution (October 1917) created the world’s first communist state. The fear of communist revolution spread across Europe, contributing to the conditions that produced fascism in Italy (1922) and Germany (1933)
• Weakening of democracy: The League of Nations, intended to maintain peace, was fatally weakened by the US Senate’s refusal to ratify the Treaty of Versailles. The democratic governments of Weimar Germany, Liberal Italy, and the Spanish Republic all eventually collapsed under the pressures generated by WWI’s aftermath
• Mandates system: The British and French empires expanded through the mandate system — the former colonies of Germany and the Ottoman Empire in Africa and the Middle East were assigned as League of Nations mandates to be administered by the victors. This planted seeds of later conflicts (Palestine, Iraq, Syria)

Economic Effects

• War debt: Britain owed the USA $4.7 billion; France$3.9 billion; total Allied war debt approximately $22 billion
• German reparations: The Dawes Plan (1924) and Young Plan (1929) restructured German reparations payments, creating a circular dependency: US banks lent money to Germany → Germany paid reparations to France and Britain → France and Britain repaid war debts to the USA. The Great Depression destroyed this system in 1929
• Inflation: Germany’s hyperinflation (1923) wiped out middle-class savings, generating lasting resentment that Hitler could exploit
• Structural damage: France and Belgium suffered the greatest physical destruction — the Western Front had crossed some of Europe’s most industrialised regions

Social Effects

• Women’s suffrage: Women who had worked in industry and nursing during the war received the vote in Britain (1918, limited), Germany (1919), USA (1920), and elsewhere. The war accelerated — though did not cause — women’s political enfranchisement
• Changing class relations: The shared experience of the trenches challenged deference to traditional elites; officers and men could no longer be kept in separate social worlds when sharing the same mud
• Shell shock and trauma: The war produced the first large-scale recognition of psychological casualties (then called “shell shock,” now understood as post-traumatic stress disorder). Over 80,000 British cases were officially diagnosed; actual numbers were far higher
• Colonial consequences: Non-European soldiers from India, West Africa, Indochina, and the Arab world served in European armies. Their service created expectations of political reform that colonial powers would spend the next three decades resisting — contributing to independence movements

Demographic Effects

• Total military dead: approximately 10 million
• Total civilian dead: approximately 7 million
• Influenza pandemic (1918–1919): killed an estimated 50–100 million people worldwide — far more than the war itself
• Refugee populations: the collapse of empires produced large-scale population displacement, particularly among Armenian survivors of the genocide, Greeks expelled from Anatolia, and Jews fleeing pogroms in the former Russian Empire

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3.3 Effects of World War II

The Peace Settlements: No Single Treaty

Unlike WWI, WWII produced no single comprehensive peace conference. The absence of a German peace treaty until the Two-Plus-Four Agreement (1990) reflected the Cold War’s immediate onset. Key settlements:

Europe:

• Potsdam Conference (July–August 1945): Germany divided into four occupation zones (American, British, French, Soviet); German borders shifted west (Oder-Neisse line); 12–14 million ethnic Germans expelled from Eastern Europe; German military disbanded
• No Japanese peace treaty until 1951 (San Francisco Treaty): Japan renounced all overseas territories; US occupation oversaw democratisation and constitutional reform; Japanese rearmament was forbidden under Article 9 of the 1947 constitution
• Creation of the United Nations (October 1945): Replaced the failed League of Nations; the Security Council gave the five permanent members (USA, USSR, UK, France, China) veto power — a realistic recognition that collective security required great power cooperation

Political Effects

• German division: Germany was divided into West Germany (Federal Republic, 1949) and East Germany (German Democratic Republic, 1949) — reflecting the Cold War division of Europe along the Iron Curtain
• Soviet expansion: The USSR established satellite states across Eastern Europe (Poland, Czechoslovakia, Hungary, Romania, Bulgaria, East Germany) — creating the Eastern Bloc
• Decolonisation: The war fatally weakened European colonial empires. Britain, France, the Netherlands, and Belgium had all either been defeated or severely weakened; the moral credibility of colonial rule collapsed in the aftermath of a war fought (officially) for freedom and democracy. India gained independence in 1947; the French empire began its long dissolution
• The Cold War: The wartime alliance between the USA and USSR collapsed almost immediately. The Truman Doctrine (1947) committed the USA to containing Soviet expansion; the Marshall Plan (1948) rebuilt Western European economies to resist communism; the Berlin Blockade (1948–1949) and NATO (1949) formalised the division of Europe

Economic Effects

• US economic dominance: The USA was the only major power whose homeland was undamaged. US GDP grew during the war; by 1945 the USA produced approximately 50% of world industrial output. The Bretton Woods system (1944) established the US dollar as the world’s reserve currency
• European devastation: Total destruction in Germany, Poland, the USSR, and much of Eastern Europe. Soviet GDP fell by 25% during the war; approximately 1,700 Soviet towns and 70,000 villages were destroyed; 25 million Soviet citizens were left homeless
• Marshall Plan: The US committed $13 billion (approximately$140 billion in current dollars) to rebuilding Western Europe, 1948–1952. Western European economic recovery was rapid; the contrast with Soviet-controlled Eastern Europe contributed to the Cold War’s ideological competition
• Japanese economic transformation: US-directed reconstruction of Japan created the conditions for the Japanese “economic miracle” — 10% annual GDP growth rates in the 1950s–1970s

Social Effects

• Women’s roles: Women’s wartime industrial and military roles were partially reversed after 1945 — the “baby boom” and suburban domesticity reasserted traditional gender norms in the USA and Western Europe — but the precedent of female industrial and professional capability had been established and could not be fully suppressed
• Holocaust memory: The Nuremberg Trials (1945–1946) established the principle of individual criminal responsibility for war crimes and crimes against humanity. The Universal Declaration of Human Rights (1948) and the Genocide Convention (1948) were direct responses to the Holocaust. Holocaust memory became a central feature of 20th-century European identity
• Veteran experience: Returning veterans in the USA, Britain, and elsewhere expected social rewards for service. The GI Bill in the USA (1944) subsidised university education, home ownership, and business loans for veterans — accelerating middle-class expansion

Demographic Effects

• Total dead: estimated 70–85 million (the deadliest conflict in human history)
• Military dead: approximately 22 million
• Civilian dead: approximately 38–55 million (including 6 million Jews murdered in the Holocaust, approximately 7 million Soviet civilians, approximately 15–20 million Chinese)
• Displaced persons: approximately 60 million Europeans displaced by the end of the war
• The Holocaust: 6 million Jews — approximately two-thirds of European Jewry — murdered; Roma, disabled people, and others also targeted
• Hiroshima and Nagasaki: approximately 110,000–210,000 dead at the time; long-term radiation casualties added tens of thousands more

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3.4 Effects of the Spanish Civil War (1936–1939)

Political effects:

• Franco’s victory established a right-wing authoritarian dictatorship that lasted until 1975 — making Spain the longest-surviving fascist-era regime in Europe
• The Republic’s defeat discredited the Popular Front strategy of communist-socialist-liberal cooperation and contributed to the breakdown of European antifascist unity
• The International Brigades’ experience radicalised a generation of leftists and writers (George Orwell’s Homage to Catalonia; Ernest Hemingway’s For Whom the Bell Tolls)

International effects:

• Germany and Italy demonstrated the effectiveness of mechanised warfare and strategic bombing — lessons applied in WWII (Blitzkrieg, aerial bombardment)
• The Spanish Civil War was a “dress rehearsal” for WWII: the same ideological fault lines (fascism vs. democracy/communism), the same great powers, many of the same weapons systems
• Soviet intervention bought arms influence at the cost of alienating Western governments and giving the right-wing narrative that the Republic was a communist puppet

Demographic effects:

• Approximately 500,000 dead (military and civilian)
• Approximately 500,000 Spanish Republicans fled into exile in France after Franco’s final offensive (January–March 1939)
• Tens of thousands imprisoned, executed, or subjected to forced labour under Franco’s postwar repression

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3.5 Effects of the Chinese Civil War (1927–1949)

Political effects:

• The People’s Republic of China was proclaimed on October 1, 1949. China’s political direction for the next 70+ years was set: a one-party communist state under the CCP
• The KMT established the Republic of China on Taiwan — creating the cross-strait division that remains unresolved today
• China entered the Korean War in October 1950, demonstrating that it would actively defend its borders and regional interests — transforming the Cold War in Asia

Economic and social effects:

• Land reform redistributed land from landlords to peasants across the mainland — the greatest redistribution of property in human history in terms of the number of people involved
• The CCP’s campaigns against “class enemies” (landlords, capitalists, “rightists”) killed an estimated 1–2 million people in the years immediately after 1949
• The consolidation of CCP power set the conditions for the Great Leap Forward (1958–1962) — Mao’s disastrous industrialisation campaign that caused famine deaths estimated at 15–55 million

Demographic effects:

• The civil war itself (1927–1949) killed an estimated 8 million people; the overlapping Sino-Japanese War added millions more
• Approximately 1.5 million KMT soldiers and civilians fled to Taiwan
• The CCP’s victory ended centuries of landlord dominance in the Chinese countryside — transforming the social structure of the world’s most populous country

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Section 4: Comparative Analysis

4.1 Comparing Causes Across Wars

4.2 Comparing Effects Across Wars

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Section 5: Essay Skills for Topic 11

5.1 Structuring a Causes Essay

A strong thesis for a causes essay will:

1. Directly address the specific claim in the question (agree, partially agree, or disagree)
2. Identify the two or more cases you will use (from different regions if the question requires it)
3. Signal your analytical position — which type of cause you consider most significant and why

Weak thesis: “There were many causes of WWI including militarism, alliances, imperialism, and nationalism.”

Strong thesis: “While the long-term structural causes — the alliance system, arms race, and great-power imperial rivalry — created the conditions in which a general European war was possible, the specific decisions taken during the July Crisis of 1914, particularly Germany’s ‘blank cheque’ and Austria-Hungary’s deliberate use of an unacceptable ultimatum, were the decisive factors that converted a regional assassination into a world war.”

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5.2 Structuring an Effects Essay

A strong effects essay will:

1. Distinguish between short-term and long-term effects (immediate aftermath vs. consequences that unfolded over decades)
2. Address multiple categories of effects (political, economic, social, demographic) rather than treating all effects as equal
3. Evaluate which effects were most significant and why — not all effects are equally important

The “unintended consequences” argument:

Many of the most significant effects of 20th-century wars were not foreseen by the parties who started them. Germany did not expect that WWI would produce the Bolshevik Revolution; Britain and France did not expect that WWII would accelerate the decolonisation of their empires; Spain’s military conspirators did not expect a three-year civil war. Identifying unintended consequences shows examiner-level analytical sophistication.

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5.3 Cross-Region Pairings

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Section 6: Practice Questions

6.1 Paper 2 Style Questions — Full List

The following questions are modelled directly on past and specimen IB History Paper 2 questions for Topic 11. Practise these under timed conditions (40 minutes each).

Causes questions:

1. “Long-term causes were more important than short-term causes in bringing about 20th-century wars.” Discuss with reference to two wars.

2. To what extent were economic factors responsible for the outbreak of one 20th-century war?

3. Compare and contrast the causes of two 20th-century wars from different regions.

4. “The failures of diplomacy were the main cause of 20th-century wars.” To what extent do you agree? Refer to two wars in your answer.

5. Evaluate the role of ideology in causing two 20th-century wars.

Practices questions:

6. Examine the role of technology in changing the nature of warfare in one 20th-century war.

7. “Civilians suffered more than soldiers in 20th-century wars.” To what extent do you agree? Refer to two wars.

8. Compare and contrast the role of civilians in two 20th-century wars.

9. To what extent was the use of air power decisive in one 20th-century war?

10. Evaluate the significance of resistance movements in two 20th-century wars.

Effects questions:

11. “Peace settlements after 20th-century wars created more problems than they solved.” Discuss with reference to two peace settlements.

12. Compare and contrast the political effects of two 20th-century wars.

13. To what extent did 20th-century wars lead to significant social change? Refer to two wars.

14. Examine the demographic consequences of one 20th-century war.

15. “The economic effects of 20th-century wars were felt most severely by the defeated powers.” To what extent do you agree? Refer to two wars.

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6.2 Model Answer Outlines

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Section 7: Key Timelines

World War I — Master Chronology

World War II — Master Chronology

Spanish Civil War — Master Chronology

Chinese Civil War — Master Chronology

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Quick Recall — Section 1 (Causes)

Try to answer without scrolling up:

1. What is the difference between long-term, short-term, and immediate causes?
2. What were the four long-term causes of WWI (MAIN)?
3. What was the “blank cheque” and why was it significant?
4. Why did the Shanghai Massacre (1927) cause the Chinese Civil War to resume?

Reveal answers

1. Long-term causes are structural conditions that develop over decades and make war possible; short-term causes are specific tensions that develop in the months or years before the war and make it likely; immediate causes are the specific triggers (the spark) that produce the outbreak in days or weeks.
2. Militarism (arms race, rigid war plans), Alliance systems (Triple Alliance vs. Triple Entente), Imperialism (colonial rivalry), Nationalism (pan-Slavism, pan-German nationalism).
3. Germany’s unconditional promise of support to Austria-Hungary (July 5–6, 1914), allowing Austria-Hungary to pursue war with Serbia without fear of German withdrawal. Without the blank cheque, Austria-Hungary would likely have backed down when Russia mobilised — it converted a bilateral dispute into a European war.
4. Chiang Kai-shek ordered the massacre of thousands of CCP activists and workers in Shanghai (April 12, 1927), ending the First United Front and converting the uneasy KMT-CCP political alliance into open military conflict that continued (with a brief interruption during WWII) until 1949.

Quick Recall — Section 2 (Practices)

Try to answer without scrolling up:

1. What is “total war” and which 20th-century conflicts best exemplify it?
2. Why did the Western Front descend into trench warfare in 1914?
3. What was the Guernica bombing and why is it historically significant?
4. How did WWII change the role of civilians compared to WWI?

Reveal answers

1. Total war is a conflict in which the entire society and economy are mobilised for the war effort and the distinction between combatants and civilians is eroded. WWI and WWII are the clearest 20th-century examples.
2. The failure of Germany’s Schlieffen Plan at the Battle of the Marne (September 1914) halted the German advance. Both sides dug defensive positions stretching from the Channel to Switzerland. The combination of machine guns, artillery, and barbed wire made offensive operations so costly that a static trench line was maintained for four years.
3. The Condor Legion (Germany) and Aviazione Legionaria (Italy) bombed the Basque town of Guernica on April 26, 1937, killing hundreds of civilians. It was historically significant as the first major aerial bombardment of a civilian town, demonstrating the new use of air power against non-combatants. Picasso’s painting made it a symbol of modern warfare’s inhumanity.
4. In WWII civilian deaths (38–55 million) exceeded military deaths (approximately 22 million) for the first time, through the Holocaust, strategic bombing, and occupation policies. In WWI, military deaths significantly outnumbered civilian deaths (10 million vs. 7 million).

Quick Recall — Section 3 (Effects)

Try to answer without scrolling up:

1. What were the five Paris Peace Conference treaties and which defeated power did each address?
2. What political system did China adopt after 1949 and what was the significance?
3. What were the Nuremberg Trials and what new legal principle did they establish?
4. How did WWI affect women’s political rights?

Reveal answers

1. Versailles (Germany), Saint-Germain (Austria), Trianon (Hungary), Neuilly (Bulgaria), Sevres/Lausanne (Ottoman Empire/Turkey).
2. China became a one-party communist state under the Chinese Communist Party (CCP), proclaimed as the People’s Republic of China on October 1, 1949. This set China’s political direction for the next 70+ years and began the cross-strait division with Taiwan that remains unresolved.
3. The Nuremberg Trials (1945–1946) tried senior Nazi leaders for war crimes, crimes against peace, and crimes against humanity. The new legal principle was individual criminal responsibility — leaders could be held personally accountable for state-sanctioned atrocities and could not use “following orders” as a complete defence.
4. Women who had contributed to the war effort (in industry and nursing) received the vote in Britain (1918, limited), Germany (1919), and the USA (1920). WWI accelerated women’s political enfranchisement, though it did not cause it — the suffrage movements had existed for decades before the war.

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Watch: World War I — Causes and Outbreak

TED-Ed · 6 min · The War That Changed the World — causes of WWI from assassination to continental war

OverSimplified · 24 min · World War I explained — causes, key battles, trench warfare, and the aftermath

Watch: World War II — Causes and Road to War

OverSimplified · 25 min · How the world stumbled into WWII — Treaty of Versailles, rise of fascism, appeasement, and the triggers of global conflict

Watch: Practices of War — Technology and Civilians

TED-Ed · 5 min · How did WWI change modern warfare? — from trench warfare to tanks, gas, and air power

Watch: Effects and Results — Peace Settlements and the Post-War World

TED-Ed · 6 min · Who won WWI? — why the peace settlement planted the seeds of the next war

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Section 8: Key Historians and Historiography