Causes of World War II

The political, economic, ideological, and military developments that led to World War II’s outbreak in 1939, including aggression by Germany, Italy, and Japan and the failure of collective security.

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Nazi Germany: The totalitarian German state ruled by Adolf Hitler from 1933 until its defeat in 1945. Its rearmament and territorial expansion drove the European crisis toward war.
Treaty of Versailles: The 1919 peace treaty that imposed territorial, military, and financial terms on Germany after World War I. Its restrictions and territorial losses became powerful targets for German nationalist politics.
Adolf Hitler: The leader of Nazi Germany from 1933 to 1945 and its dictator from 1934. His pursuit of territorial expansion and racial empire drove Germany’s escalating aggression.
Collective security: An international system in which states cooperate to deter or respond to aggression against any member. The League’s collective-security commitments failed to restrain aggressors in the 1930s.
Invasion of Poland: Germany’s attack on Poland beginning on 1 September 1939, followed by the Soviet invasion from the east on 17 September. The attack prompted Britain and France to declare war on Germany.
Appeasement: A policy of conceding demands to an aggressor in the hope of preventing conflict. British and French concessions failed to stop Hitler’s expansion and weakened deterrence.
World War I: A global conflict fought mainly in Europe from 1914 to 1918 among the Allied and Central Powers. Its destruction and unresolved political disputes shaped the postwar order that later broke down.
Neville Chamberlain: British prime minister from 1937 to 1940, associated with the policy of appeasement toward Germany. His government negotiated the Munich Agreement while seeking to avert another European war.
League of Nations: An international organization founded in 1920 to promote cooperation and prevent war. Its inability to enforce decisions against Japan, Italy, and Germany weakened confidence in peaceful remedies.
Declaration of war by the United Kingdom and France: The declarations of war against Germany made by the United Kingdom and France on 3 September 1939. They converted Germany’s invasion of Poland into a wider European war.