What caused World War 2? The road to war, explained

What caused World War 2 was, above all, the decision of three expansionist regimes, Nazi Germany, Fascist Italy and militarist Japan, to take territory by force, and the failure of other powers to stop them while it was still cheap to do so. Behind those decisions lay the resentments left by World War I and the economic collapse of the Great Depression, which helped bring those regimes to power and made others reluctant to fight.

In short

• The causes come in layers: a resented peace settlement, a world depression, aggressive dictatorships, and a policy of giving way to them.
• Full-scale war began in Asia with Japan's invasion of China in 1937, and in Europe with Germany's invasion of Poland on 1 September 1939.
• Historians agree Hitler's aims were central in Europe; they still argue about how far he planned the war, and whether appeasement could have stopped it.

What did World War I leave behind?

The first layer is the peace that ended World War I. The Treaty of Versailles, signed on 28 June 1919, took away about 13% of Germany's prewar territory and some 10% of its people, according to the United States Holocaust Memorial Museum. It capped the German army at 100,000 men, banned an air force, kept the Rhineland free of troops, and in Article 231, the "war guilt clause", made Germany accept responsibility for the war's damage, the basis for reparations.

Germans of almost every party called it a Diktat, a dictated peace. Right-wing parties made overturning it a central promise, helped by the false "stab-in-the-back" myth that the army had not been beaten but betrayed at home. Italy, on the winning side, felt cheated of the land it had been promised. Japan, also a winner, resented the Western powers' refusal to accept a racial equality clause in the League of Nations covenant. The League itself, set up to keep the peace, had no army, and the United States never joined.

How did the Great Depression make war more likely?

The second layer is economic. The Great Depression that began in 1929 threw tens of millions out of work. In Germany, unemployment passed 6 million by early 1932, and voters deserted the parties of the Weimar Republic for the extremes. That collapse is how Hitler came to power in January 1933, at the head of a movement that wanted to tear up Versailles and conquer "living space" in eastern Europe.

The slump hit Japan too, through falling silk and rice prices, and strengthened army officers who argued that an empire in mainland Asia would give Japan the raw materials and markets it lacked. Across the democracies, it drained budgets and the will to rearm. Hard times didn't make war inevitable, but they put aggressive leaders in charge and left their neighbors less able to answer them.

What did Germany, Italy and Japan do?

The third layer is a series of acts of aggression, each testing whether anyone would respond. The dates below come from the Holocaust Memorial Museum's key dates and the U.S. State Department's Office of the Historian.

Date	What happened	The response
18 September 1931	Japan seizes Manchuria after a staged railway explosion near Mukden	The League condemns it; Japan walks out of the League in 1933
1933 to 1935	Germany leaves the League and openly rearms, bringing back conscription	Protests, no action
October 1935	Italy invades Ethiopia	Weak League sanctions, never applied to oil
March 1936	German troops reoccupy the Rhineland, breaking Versailles	Britain and France do not resist
1936 to 1939	Germany and Italy back Franco's side in the Spanish Civil War	Britain and France stay out
7 July 1937	Japan begins full-scale war on China after a clash near Beijing	The war in Asia has begun
March 1938	Germany absorbs Austria, the Anschluss	No resistance
29 September 1938	The Munich agreement hands Czechoslovakia's Sudetenland to Germany	Britain's Neville Chamberlain hails "peace for our time"
March 1939	Germany occupies the rest of the Czech lands	Britain and France promise to defend Poland
23 August 1939	Germany and the Soviet Union sign a non-aggression pact, secretly splitting eastern Europe	Hitler no longer fears a two-front war
1 September 1939	Germany invades Poland	Britain and France declare war on 3 September

Read down the right-hand column and the pattern is plain: until 1939, each step met words rather than force. Italy's Mussolini, in power since 1922, and Hitler drew together in the Rome–Berlin Axis in 1936, the same year Germany and Japan signed a pact against international communism. All three were regimes built on fascist or militarist ideas that treated war as a test of national strength.

Why did Britain and France appease Hitler?

The fourth layer is appeasement, the policy of meeting a dictator's demands in the hope of satisfying him. It came from real pressures, not just weakness of will. Britain and France had lost millions in World War I, and their voters dreaded another. Their armed forces had been cut in the 1920s and were not ready. Many in Britain thought Versailles had been too harsh and that some German claims, such as the Rhineland or the German-speaking Sudetenland, were fair. Some leaders saw Soviet communism as the greater danger.

Munich ended that hope. When Hitler took Prague in March 1939, a city with no German majority, it showed his aims went beyond undoing Versailles. Britain and France then guaranteed Poland, and when Germany invaded it they declared war, though they could do little to save it. The Soviet Union invaded Poland from the east on 17 September, as the secret protocol had arranged.

Still argued

Three debates continue. Did Hitler plan the war? In 1961 the British historian A. J. P. Taylor argued that he was an opportunist who stumbled into a war over Poland; most historians since, drawing on Nazi documents, see a long-held aim of conquest in the east, with the timing set by opportunity. Was appeasement a mistake? The verdict of 1940, that it was a shameful failure, has been challenged by historians who argue that Britain needed the time to rearm, especially its air defenses; critics answer that Germany gained more from the delay. And when did the war begin? Many historians of Asia date it from 1937, or even 1931, rather than 1939.

What happened in the war, in one paragraph?

Germany overran Poland in weeks and, in 1940, Denmark, Norway, the Low Countries and France, leaving Britain fighting alone. On 22 June 1941 it invaded the Soviet Union, and behind its armies began the systematic murder of Europe's Jews, the Holocaust, in which about 6 million Jews were killed. On 7 December 1941, Japan attacked Pearl Harbor, bringing the United States into the war. The tide turned in 1942 and 1943 at Midway, El Alamein and Stalingrad. Allied forces landed in Normandy on 6 June 1944, and Germany surrendered on 8 May 1945. After the United States dropped atomic bombs on Hiroshima and Nagasaki and the Soviet Union declared war on Japan, Japan surrendered, signing on 2 September 1945. Estimates of the dead vary widely; the National WWII Museum gives about 15 million military and 45 million civilian deaths, and notes China's civilian toll may have been far higher. The war's end divided Europe and began the Cold War.

Historians weigh these causes differently, which is the ordinary work of history. Few think any one layer would have been enough alone. A resented peace and a depression created the conditions; particular leaders chose war; and the powers that might have stopped them chose not to until it was too late to do so cheaply.

Check yourself

• Name the four layers of causes, and say how the Great Depression connects the first to the third.
• Why did Britain and France give way at Munich, and what changed their minds six months later?
• Why do some historians say World War II began in 1937, not 1939?

Sources

• Treaty of Versailles — United States Holocaust Memorial Museum, Holocaust Encyclopedia
• World War II: Key Dates — United States Holocaust Memorial Museum, Holocaust Encyclopedia
• The Mukden Incident of 1931 and the Stimson Doctrine — Office of the Historian, U.S. Department of State
• The Versailles Treaty, June 28, 1919 — The Avalon Project, Yale Law School
• Research Starters: Worldwide Deaths in World War II — The National WWII Museum