History · My World Quest — full scheme of work · pioneer
Why Did the World Go to War — Again?
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For grown-ups
Companion summary
What if one leader's decisions could reshape every single border on the entire planet?
Understanding these causes helps you make sense of today's news, treaties, and why the United Nations even exists.
Over 70 million people were affected — yet it began with tensions that built quietly over two decades.
When you finish, try mapping which countries changed borders after 1945 and ask why.
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A World That Promised 'Never Again'
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The First World War ended in 1918. Millions had died. World leaders signed a peace treaty and declared it the 'war to end all wars.' Yet just twenty-one years later, in September 1939, the world was at war again. How does that happen? Think about this: if you settled an argument by humiliating someone and taking their belongings, what might they do when they grew stronger? Keep that thought in mind as you work through this lesson.
💭 Think about this
What do you already know about how or why the Second World War began? Jot down one cause you have heard of — even if you are not sure it is correct.
✨
A World That Promised 'Never Again'
⚔️🏰🛡️🎖️
The First World War ended in 1918. Millions had died. World leaders signed a peace treaty and declared it the 'war to end all wars.' Yet just twenty-one years later, in September 1939, the world was at war again. How does that happen? Think about this: if you settled an argument by humiliating someone and taking their belongings, what might they do when they grew stronger? Keep that thought in mind as you work through this lesson.
💭 Think about this
What do you already know about how or why the Second World War began? Jot down one cause you have heard of — even if you are not sure it is correct.
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Causes and Consequences: Unpicking a Global Catastrophe
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**The Causes of the Second World War**
Historians usually point to five interconnected causes. None worked alone — they formed a chain.
**1. The Treaty of Versailles (1919)**
After the First World War, Germany was forced to sign the Treaty of Versailles. It made Germany accept full blame for the war, pay enormous (financial penalties), lose territory, and severely limit its armed forces. Many Germans felt deeply humiliated and economically crushed. This resentment became fuel for extreme politics.
**2. The Rise of s**
Economic collapse across Europe — made far worse by the Great Depression from 1929 — pushed desperate populations toward extreme leaders who promised easy answers. In Germany, Adolf Hitler and the Nazi Party rose to power in 1933. In Italy, Benito Mussolini had been in power since the 1920s. In Japan, militarist leaders expanded aggressively across Asia. These regimes were totalitarian: they demanded total control over citizens and suppressed all opposition.
**3. **
Britain and France hoped to avoid another war by making concessions — giving in to some of Hitler's demands. This policy is called appeasement. In 1938, they allowed Hitler to take the Sudetenland region of Czechoslovakia, believing he would stop there. He did not. Appeasement encouraged further aggression.
**4. The Failure of the League of Nations**
After 1918, countries formed the League of Nations to settle disputes peacefully. But the USA never joined, and the League had no army to enforce its decisions. When Japan invaded Manchuria (1931) and Italy invaded Abyssinia (Ethiopia, 1935), the League was powerless. This signalled that aggression would not be effectively punished.
**5. Hitler's Ideology and Expansionism**
Hitler openly stated his goal: to create a vast German empire (Lebensraum, or 'living space') and to destroy groups he considered inferior, especially Jewish people. His invasion of Poland on 1 September 1939 triggered Britain and France to declare war on Germany — the formal start of the Second World War in Europe.
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**The Consequences of the Second World War**
The war lasted from 1939 to 1945 and affected every continent. Its consequences reshaped the modern world.
**1. Human Cost**
An estimated 70–85 million people died — the deadliest conflict in history. Around half of all deaths were civilians, not soldiers. The Holocaust — Nazi Germany's systematic murder of six million Jewish people, along with millions of others including Roma, disabled people, and political prisoners — stands as one of history's worst s.
**2. **
European empires had used colonial troops from Africa, Asia, and the Caribbean to fight the war. Afterward, those nations could no longer claim moral authority to rule others. Independence movements accelerated across Asia and Africa. India gained independence in 1947; many African nations followed in the 1950s and 1960s.
**3. The United Nations (1945)**
To replace the failed League of Nations, world leaders founded the United Nations in 1945. Its purpose was to prevent future wars through international cooperation, diplomacy, and shared rules. Today it has 193 member states.
**4. The **
The USA and the Soviet Union emerged as the two dominant 's.' Though they had been allies against Hitler, their opposing ideologies — capitalism versus communism — quickly created deep rivalry. This tension, known as the Cold War, shaped global politics for the next four decades.
**5. Economic Rebuilding and the Marshall Plan**
Much of Europe and Asia lay in ruins. The USA launched the Marshall Plan (1948), providing over $13 billion to rebuild Western European economies. This helped prevent further political collapse and bound Western nations more closely to American influence.
Worked example
**Cause–Consequence Chain: From Treaty to War**
Here is how to trace a chain of causation:
→ The Treaty of Versailles (1919) humiliated Germany and wrecked its economy.
→ Economic misery worsened by the Great Depression (1929) made Germans desperate for change.
→ Hitler and the Nazi Party exploited that desperation, gaining power in 1933.
→ Britain and France adopted appeasement, allowing Hitler to expand without immediate punishment.
→ Hitler invaded Poland on 1 September 1939.
→ Britain and France declared war on Germany on 3 September 1939.
This chain shows that the causes are not isolated events — each one made the next more likely. When you answer a question about causes, always try to show how they link together, not just list them separately.
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Test Your Understanding
Try it yourself
Read each question carefully. Choose the best answer. Then check the explanation to see how well your reasoning matched.
1. The Treaty of Versailles contributed to the outbreak of the Second World War mainly because it:
2. Why did the policy of appeasement fail to prevent the Second World War?
3. Which of the following is a consequence, not a cause, of the Second World War?
4. How did the Second World War accelerate decolonisation across Asia and Africa?
5. A student argues: 'Hitler's personal ambitions were the single most important cause of the Second World War.' Do you agree? Explain your reasoning, using at least two other causes to support your answer.
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Bringing It Together
The Second World War had multiple connected causes — including the Treaty of Versailles, the Great Depression, totalitarian regimes, appeasement, and Hitler's ideology — no single cause works alone.
The war's consequences reshaped the entire world: an estimated 70–85 million people died, the Holocaust was one of history's worst genocides, and new institutions like the United Nations were created.
Causation in history is a chain: understanding how events link together is more powerful than simply listing facts.
1. Which consequence of the Second World War do you think most changed the world in the long run — and why?
🌱 Reflect
Think about the chain of causes you studied today. Could the Second World War have been prevented at any point in that chain? Which moment do you think was the turning point where a different decision could have changed history?
