The Second World War (1939–1945) was the most devastating conflict in human history. More than 70 million people died, entire cities were destroyed, and the political map of the world was transformed. For GCSE and A-Level history students, one of the key questions is: “What caused the Second World War?”
The answer, as with the First World War, is complex. The conflict did not begin with a single event, but with a series of long-term tensions, short-term crises and choices made by leaders that escalated into a global catastrophe.
This blog explains the causes of WW2 in a way that is engaging and designed around common questions.
What Were the Main Causes of the Second World War?
Historians point to several key causes of WW2, often grouped into:
- The Treaty of Versailles – how the settlement after WW1 created anger and instability.
- The rise of Hitler and Nazi ideology – expansion, racism and dictatorship.
- Appeasement – Britain and France’s attempts to avoid conflict, which instead encouraged aggression.
- The failure of the League of Nations – international weakness in the 1930s.
- The Nazi–Soviet Pact – a shocking alliance that cleared the way for war.
- American intervention in 1941 – which turned a European war into a truly global conflict.
Each of these factors played a role in shaping the world between 1919 and 1939, with 1941 marking the point where the balance of power shifted.
How Did the Treaty of Versailles Cause the Second World War?
Signed in 1919, the Treaty of Versailles ended the First World War but left deep scars:
- War guilt clause (Article 231): Germany was forced to accept full responsibility for causing WW1.
- Reparations: Germany was ordered to pay £6.6 billion, crippling its economy.
- Military restrictions: The German army was limited to 100,000 men, with no tanks, aircraft, or submarines.
- Loss of territory: Alsace-Lorraine returned to France; Poland gained land; overseas colonies were taken away.
Many Germans felt the treaty was unfair and humiliating. The resentment it created was a fertile ground for extremist politics, especially the rise of Adolf Hitler, who promised to overturn Versailles.
How Did the Rise of Hitler Lead to the Second World War?
Adolf Hitler, leader of the Nazi Party, came to power in 1933. His aims, set out in Mein Kampf and Nazi policy, included:
- Destroying the Treaty of Versailles – rebuilding Germany’s army and pride.
- Lebensraum (“living space”): Expanding eastwards to gain land for Germans.
- Racial ideology: Belief in German racial superiority and hostility towards Jews and Slavs.
- Reuniting German-speaking peoples: Annexing Austria and other regions.
Step by step, Hitler broke international agreements:
- 1935: Reintroduced conscription and built up the air force (Luftwaffe).
- 1936: Remilitarised the Rhineland.
- 1938: Annexed Austria (the Anschluss).
- 1938: Demanded the Sudetenland from Czechoslovakia.
Each success made him more confident that Britain and France would not stop him.
Why Did Appeasement Fail?
Appeasement was the policy followed by Britain (led by Neville Chamberlain) and France in the 1930s: giving in to Hitler’s demands to avoid war.
Reasons for appeasement included:
- Memories of the horrors of the First World War.
- Belief that the Treaty of Versailles had been too harsh.
- Economic weakness after the Great Depression.
- Hope that Germany would be a barrier against Soviet communism.
The most famous moment of appeasement was the Munich Agreement (1938), when Britain and France allowed Hitler to take the Sudetenland. Chamberlain returned and declared “peace for our time”, but within a year, Hitler had seized the rest of Czechoslovakia.
Appeasement failed because it convinced Hitler that the Allies were weak and unwilling to resist.
How Did the Failure of the League of Nations Contribute to WW2?
The League of Nations, created in 1919 to maintain peace, proved too weak to prevent aggression.
- 1931: Japan invaded Manchuria – the League failed to act decisively.
- 1935: Italy, led by Mussolini, invaded Abyssinia (Ethiopia) – sanctions were ineffective.
- 1930s: Germany ignored the League by rearming and remilitarising.
The League lacked an army, relied on unanimous decisions and was undermined when powerful countries like the USA stayed out. Its weakness showed dictators that aggression would not be punished.
What Was the Nazi–Soviet Pact and Why Was It Important?
On 23 August 1939, Hitler and Stalin shocked the world by signing the Nazi–Soviet Pact (also called the Molotov–Ribbentrop Pact).
- Publicly, it was a non-aggression pact: Germany and the Soviet Union promised not to attack each other.
- Secretly, they agreed to divide Poland and other parts of Eastern Europe between them.
The pact cleared the way for Hitler to invade Poland without fear of a war on two fronts.
On 1 September 1939, Germany invaded Poland. Two days later, Britain and France declared war. The Second World War had begun.
What Was the Immediate Cause of the Second World War?
The invasion of Poland was the trigger. However, it is important to remember that it was the combination of long-term factors (Versailles, Hitler’s aims, appeasement, League failure) and short-term decisions (the Nazi–Soviet Pact) that made war inevitable by 1939.
Why Did the United States Enter the Second World War in 1941?
Although war began in Europe in 1939, the United States initially stayed out. American opinion was divided, with many still committed to isolationism after WW1. However, by December 1941, the USA entered the conflict, which was a turning point in world history.
Key reasons for US intervention:
- Support for Britain: President Franklin D. Roosevelt (FDR) admired Churchill’s resolve and gradually increased aid through the Lend-Lease Act (1941), which sent weapons, ships, and supplies to Britain.
- German aggression at sea: German U-boats attacked American shipping in the Atlantic, threatening trade and supplies.
- Pearl Harbour attack: On 7 December 1941, Japan launched a surprise attack on the US naval base at Pearl Harbour, Hawaii, killing over 2,400 Americans.
- Hitler’s declaration of war: On 11 December 1941, Hitler declared war on the USA, turning a regional conflict into a truly global one.
From this moment, the war changed dramatically. America’s industrial and military strength, combined with Britain’s resilience and the Soviet Union’s manpower, created an alliance that would eventually defeat Nazi Germany.
Could the Second World War Have Been Prevented?
Historians debate this question:
- Some argue that the harshness of Versailles and the failure of the League made war unavoidable.
- Others believe that stronger resistance to Hitler in the 1930s. For example, defending Czechoslovakia in 1938 might have prevented the conflict.
What is clear is that a mixture of resentment, dictatorship, failed diplomacy and miscalculation combined to unleash the deadliest war in history.
Why Is It Important to Study the Causes of the Second World War?
Studying WW2 teaches lessons that are still relevant today:
- The dangers of appeasing aggression.
- How economic crisis can fuel extremism.
- The cost of war in human lives.
- The importance of international cooperation to keep peace.
- The role of leadership in galvanising nations.
For GCSE and A-Level history students, the causes of WW2 are not just facts to revise. They are a reminder of how choices, reckless or wise, shape the world we live in.
“The Second World War was not simply the result of treaties signed or broken; it was shaped by human choices and decisions driven by ambition, fear, courage and caution. Leaders defined the course of history: Hitler’s reckless aggression plunged the world into catastrophe, Churchill inspired a nation through its darkest hour, and Roosevelt guided the United States from isolation to decisive action, turning the tide of war. For students, the lesson is clear: history is not predetermined. It is created by choices, sometimes reckless, sometimes courageous, that touch the lives of millions. At Apollo Scholars, we believe history offers more than knowledge; it cultivates perspective, the compass for ambition, resilience and meaningful action.”
Matt
Founder, Apollo Scholars
Conclusion: Understanding the Causes of the Second World War
The Second World War was caused by the Treaty of Versailles, the rise of Hitler, appeasement, the failure of the League of Nations, the Nazi–Soviet Pact and finally, the entry of the United States after the surprise attack on Pearl Harbour in 1941. The invasion of Poland in 1939 triggered war in Europe, but America’s intervention in 1941 transformed it into a truly global conflict.
For students, the story of WW2 is not only about tanks and battles but about the choices that led to catastrophe. By studying its causes, you gain more than exam knowledge; you gain the tools to understand the dangers of aggression, the importance of diplomacy and the responsibility we all share to prevent history from repeating itself.
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