The Causes of World War Two: A KS3 History Guide

World War Two began on 1 September 1939 when Nazi Germany invaded Poland, and Britain declared war two days later. The war did not come out of nowhere: it emerged from a chain of failures, decisions, and injustices stretching back to 1919 — and understanding those causes is one of the most important things you will study in KS3 history.

The long-term cause: the Treaty of Versailles (1919)

When World War One ended in November 1918, the victorious Allied powers — Britain, France, and the United States — met in Paris to agree on the peace terms. The resulting Treaty of Versailles, signed on 28 June 1919, imposed severe penalties on Germany:

Clause | What it meant for Germany
War Guilt Clause (Article 231) | Germany was forced to accept sole responsibility for the war
Reparations | Germany had to pay £6.6 billion in compensation (the final payment was not made until 2010)
Territory | Germany lost Alsace-Lorraine, the Polish Corridor, and all its overseas colonies
Military | The army was limited to 100,000 men; no air force; no submarines
Rhineland | The area between Germany and France was demilitarised

Many Germans regarded the treaty as a humiliation — the Diktat (dictated peace) — rather than a negotiated settlement. The economic hardship of the reparations, combined with the Great Depression from 1929, produced mass unemployment and political instability in the Weimar Republic. This environment allowed extremist parties, including the Nazi Party under Adolf Hitler, to gain popular support by promising to restore Germany's greatness and overturn the Versailles "disgrace."

The National Archives holds original documents from the Paris Peace Conference, including the terms presented to the German delegation and Germany's objections to them.

The rise of Hitler and Nazi Germany

Adolf Hitler joined the German Workers' Party in 1919 and renamed it the National Socialist German Workers' Party (the Nazis). His political ideology combined extreme German nationalism, virulent antisemitism, anti-communism, and the belief in Lebensraum — "living space" — the idea that Germany needed to expand eastwards to secure the resources it required.

After an unsuccessful armed attempt to seize power in the Munich Putsch (1923), Hitler turned to electoral politics. The Great Depression transformed the Nazis' fortunes: the party won 18% of the vote in 1930 and 37% in July 1932. Hitler was appointed Chancellor of Germany on 30 January 1933 by President Hindenburg, who fatally underestimated him. Within months, the Enabling Act (March 1933) gave Hitler dictatorial powers, and Germany became a one-party state.

From 1933, Hitler systematically broke the terms of the Treaty of Versailles:

1935: Rearmament announced publicly; conscription reintroduced

1936: German troops reoccupied the Rhineland (meant to be permanently demilitarised)

1936–1939: Germany and Italy supported Franco's Nationalist forces in the Spanish Civil War, using it as a testing ground for new weapons

1938 (March): Anschluss — Germany annexed Austria

1938 (September): Germany demanded the Sudetenland (western Czechoslovakia), which contained three million ethnic Germans

Appeasement: giving Hitler what he wanted

The British and French governments' response to Hitler's expansion in the 1930s was a policy known as appeasement — making concessions to an aggressor in the belief that satisfying their demands would prevent a larger conflict.

The most significant act of appeasement was the Munich Agreement of 30 September 1938. British Prime Minister Neville Chamberlain and French Premier Édouard Daladier met Hitler (and Mussolini) in Munich and agreed that Germany could take the Sudetenland, in exchange for Hitler's promise that he had no further territorial ambitions in Europe. Czechoslovakia was not invited to the conference at which its territory was given away. Chamberlain returned to London declaring "peace for our time."

Why did appeasement happen?

Fear of another war. The memory of the catastrophic losses of World War One (over 700,000 British dead) made British politicians and the public desperate to avoid a repeat.

Genuine belief that the grievances were partly justified. Many British politicians thought the Treaty of Versailles had been too harsh and that Hitler was correcting a genuine injustice.

Military unpreparedness. Britain's armed forces were not ready for war in 1938; buying time allowed rearmament.

Underestimation of Hitler. Many politicians genuinely believed Hitler's promises would hold.

The policy was famously challenged by Winston Churchill, who said of the Munich Agreement: "You were given the choice between war and dishonour. You chose dishonour and you will have war."

The immediate cause: the invasion of Poland (1939)

In March 1939, Hitler broke the Munich Agreement by occupying the rest of Czechoslovakia — making clear that his ambitions went far beyond uniting ethnic Germans. Britain and France responded by guaranteeing Poland's independence: if Germany attacked Poland, both countries would go to war.

On 23 August 1939, Germany and the Soviet Union signed the Nazi-Soviet Pact — a non-aggression agreement that secretly divided Eastern Europe between them and freed Hitler from the risk of a two-front war. On 1 September 1939, Germany invaded Poland from the west. Two days later, Britain and France declared war on Germany, beginning World War Two. The Soviet Union subsequently invaded Poland from the east on 17 September.

How to answer "What caused World War Two?" at KS3

At KS3, you are not simply expected to list causes — you are expected to weigh them. Historians generally identify three levels of cause:

Long-term causes: the Treaty of Versailles, the weakness of the Weimar Republic, the rise of fascism

Medium-term causes: Hitler's deliberate policy of expansion; the failures of the League of Nations

Short-term (trigger) causes: the invasion of Poland; the Nazi-Soviet Pact

An excellent KS3 essay will argue for which cause was most important and explain how the causes linked together. Was it Hitler's individual decisions that caused the war? Or would any German leader with a grievance about Versailles have done the same? Or was it British and French appeasement that gave Hitler the confidence to keep pushing? Historians still debate these questions.

Frequently asked questions

What were the main causes of World War Two?

The main causes are usually grouped as: the harsh terms of the Treaty of Versailles (1919), which humiliated Germany and created the conditions for extremism; the Great Depression from 1929, which caused mass unemployment and undermined the Weimar Republic; the rise of Adolf Hitler and Nazi Germany from 1933; the failure of the League of Nations to stop aggression; and the policy of appeasement, most notably at Munich in 1938. The immediate trigger was Germany's invasion of Poland on 1 September 1939.

What was the policy of appeasement and did it work?

Appeasement was the policy adopted by Britain and France in the 1930s of making concessions to Hitler's demands in the hope of preventing war. It failed: Hitler interpreted each concession as a sign of weakness and continued to expand. The Munich Agreement of 1938, in which the Sudetenland was given to Germany, is the most notorious example of appeasement. However, some historians argue that appeasement bought valuable time for Britain to rearm, and that in 1938 Britain genuinely was not ready for war.

Why did Britain declare war on Germany in 1939?

Britain declared war on Germany on 3 September 1939 because Germany had invaded Poland on 1 September, and Britain had guaranteed Poland's independence. After Hitler broke the Munich Agreement in March 1939 by occupying the rest of Czechoslovakia, Neville Chamberlain's government abandoned appeasement and committed to defending Poland. When Germany's invasion went ahead despite an ultimatum to withdraw, Britain and France both declared war. The decision ended the era of appeasement and marked a fundamental shift in British foreign policy.

What was the Nazi-Soviet Pact and why was it significant?

The Nazi-Soviet Pact (formally the Molotov-Ribbentrop Pact) was a non-aggression agreement signed between Germany and the Soviet Union on 23 August 1939. It included a secret protocol dividing Eastern Europe into German and Soviet spheres of influence. For Hitler, it was crucial: it meant Germany would not have to fight on two fronts when it invaded Poland. For Stalin, it appeared to buy time to build up Soviet military strength. The pact shocked the world because Nazi Germany and Communist Soviet Union were ideological enemies — but strategic interest temporarily overrode ideology.

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