Causes of World War II
The factors that ignited the largest conflict in history
The Post–WWI Peace Treaties
The Treaty of Versailles (1919) imposed heavy reparations and strict military limits on the defeated states, above all Germany. Japan — despite fighting on the winning side — was excluded from the spoils. This created a deep sense of national injustice in Germany and Japan and gave extremist movements their fuel.
The Arms Race and Military Alliances
The European states entered an intensive arms race during the 1930s, and opposing military alliances took shape: the Axis powers (Germany, Italy, Japan) on one side, and the Allied powers (Britain, France, the Soviet Union, and later the United States) on the other.
Post-WWI Economic and Social Crises
Widespread economic and social crises struck European states after World War I, culminating in the Great Depression (1929–1933). State policies on armament and trade clashed. Extremist parties exploited this disorder to seize power, as in Germany and Italy.
Britain and France's Sense of the Nazi Threat
Britain and France felt a direct threat from Nazi expansionism in Europe, especially after Germany brought several neighboring states under its influence. The two countries first tried appeasement (the 1938 Munich Agreement), but it failed and showed Hitler the West's weakness.
Germany's Annexations and Expansion in Europe
Germany annexed Austria in 1938, then pressed Poland with demands to cede the Free City of Danzig and a corridor. Germany signed a pact with the Soviet Union in 1939 to secure its eastern flank, and formed the Axis with Italy and Japan.
The Invasion of Poland and the Outbreak of War
On September 1, 1939, Germany invaded Poland. Britain and France declared war two days later, and World War II erupted, quickly spreading across the continent.