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v0.1.1485
NotesHistoryTopic 16.1Causes of the Second World War (1939–45)
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16.1.33 min read

Causes of the Second World War (1939–45)

IB History • Unit 16

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Contents

  • Why war returned in 1939
  • Ideology and the long build-up
  • Expansion, appeasement and the road to Poland

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The big idea: Just twenty years after the First World War, Europe went to war again. Why?

The short answer is that a bitter peace, a broken system for keeping order, and an economic disaster left the door open — and Adolf Hitler walked through it. Long-term resentment, ideology, and step-by-step aggression all came together in September 1939.

It helps to sort the causes the way examiners do. Some causes were long-term — deep pressures building for two decades.

Others were ideological — the beliefs that made leaders want to expand. Then came the immediate steps and the final trigger — the invasion of Poland.

A great essay separates these clearly instead of mixing them together.

  • Long-term causes — resentment at the Treaty of Versailles, the weakness of the League of Nations, and the Great Depression
  • Ideological causes — Nazi, Fascist and militarist beliefs that glorified war and empire
  • Immediate causes — Hitler's expansion (Rhineland, Austria, Czechoslovakia) and the failed policy of appeasement
  • The trigger — Germany's invasion of Poland on 1 September 1939
The Versailles wound: The 1919 Treaty of Versailles blamed Germany for the war, took away its land and colonies, limited its army, and demanded huge reparations.

Many Germans called it the diktat — a punishment forced on them. That anger became the fuel Hitler later used to win support for tearing the treaty up.
Spot it: L-I-T: Long-term (Versailles, League, Depression) · Ideological (Nazism, Fascism, militarism) · Trigger (Poland, 1939). Almost every cause you need fits one of these three groups.

A harsh peace and a weak League set the scene, but they did not make war inevitable on their own. What turned resentment into invasion was ideology — a set of beliefs, especially in Germany, that saw war and conquest as good and necessary.

  • Nazi Germany — Hitler wanted Lebensraum, the union of all German-speakers, and the destruction of the Versailles settlement
  • Italian Fascism — Mussolini dreamed of a new Roman Empire and invaded Abyssinia (Ethiopia) in 1935 to prove Italy's strength
  • Japanese militarism — army leaders sought an Asian empire, seizing Manchuria in 1931 and invading China in 1937
Why the League of Nations failed: The League was meant to stop aggression through collective security, but it had no army of its own and the USA never joined.

When Japan took Manchuria (1931) and Italy took Abyssinia (1935), the League only talked. Dictators learned a fatal lesson: aggression paid, and no one would stop them.
The Great Depression's role: The world slump after 1929 threw millions out of work and made people desperate. In Germany it destroyed trust in democracy and helped Hitler rise to power in 1933.

The Depression also made Britain and France weak and inward-looking — they were too poor and worried to rearm quickly or confront the dictators.

Long-term causes (building for years)

  • Resentment at the Treaty of Versailles (1919)
  • The weakness of the League of Nations
  • The Great Depression, which wrecked economies and democracies
  • Deep-rooted nationalism across Europe and Asia

Ideological causes (the drive to expand)

  • Nazi Lebensraum and the goal of overturning Versailles
  • Italian Fascism and dreams of a new Roman Empire
  • Japanese militarism and the hunt for resources and empire
  • A shared belief that war and conquest were legitimate

What exactly was Lebensraum?

Hitler's plan to conquer 'living space' in Eastern Europe — especially Poland and the USSR — for German settlers, seizing land and resources by force.

How did the two World Wars connect?

WWII grew directly out of WWI's unfinished business: Versailles humiliated Germany, and the failure to enforce or fairly revise it left grievances that Hitler exploited.

Was the war only about Europe?

No. Japan's expansion in Asia and the Pacific ran in parallel, and after Pearl Harbor (1941) the European and Asian wars merged into one global conflict.

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From 1936 Hitler took a series of gambles, each bolder than the last. Every time Britain and France backed down, he grew more confident.

This was the policy of appeasement — giving Hitler what he demanded, hoping each concession would be the last.

1

Rhineland, 1936

Hitler sent troops into the demilitarised Rhineland, breaking Versailles. Britain and France did nothing — the Rhineland was, after all, German land.

2

Anschluss, 1938

German troops marched into Austria and Hitler declared Anschluss. Again the West protested but did not act.

3

Sudetenland & Munich, 1938

Hitler demanded the Sudetenland. At the Munich Agreement, Britain and France handed it over to avoid war.

4

Prague, March 1939

Hitler seized the rest of Czechoslovakia — land with no German majority. This proved appeasement had failed and he wanted conquest, not just fairness.

Rhineland → Austria → Sudetenland → Prague: each step bolder, each answered with retreat.

The Munich Agreement (September 1938): At Munich, Britain's Neville Chamberlain agreed to give Hitler the Sudetenland without Czechoslovakia even being present. He returned promising 'peace for our time'.

Supporters said it bought time to rearm; critics said it was a betrayal that only made Hitler stronger and bolder.
The Nazi–Soviet Pact, August 1939: In a shock move, Nazi Germany and the Soviet Union signed the Nazi–Soviet Pact in August 1939. Two bitter enemies agreed not to fight each other.

A secret clause carved up Poland between them. This freed Hitler to attack Poland without fear of a war on two fronts — the final green light.

On 1 September 1939, Germany invaded Poland. This time Britain and France had promised to defend Poland, and appeasement was over.

Two days later, on 3 September 1939, they declared war on Germany. The Second World War in Europe had begun.

DateEventWhy it mattered
1936Remilitarisation of the RhinelandBroke Versailles; West did nothing
1938Anschluss with AustriaGermany grew stronger, unopposed
Sept 1938Munich Agreement (Sudetenland)Appeasement at its peak
Aug 1939Nazi–Soviet PactRemoved the two-front danger
1 Sept 1939Invasion of PolandThe trigger for war
3 Sept 1939Britain & France declare warWWII begins in Europe

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Related History Topics

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16.1.1Types and causes of war: the framework
16.1.2Causes of the First World War (1914–18)
16.2.1How wars are fought: the framework
16.2.2Practices of the First World War (1914–18)
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