What caused World War II?
World War II (1939–1945) was caused by the aggressive expansionism of Nazi Germany, fascist Italy, and imperial Japan, combined with the failure of Western democracies to confront aggression early through a policy of appeasement. The punitive Treaty of Versailles, the Great Depression, and the rise of totalitarian regimes created the conditions for the deadliest conflict in human history.
World War II was the most destructive conflict in human history, killing an estimated 70–85 million people. Its causes were rooted in the unresolved tensions of World War I, the economic devastation of the Great Depression, and the rise of aggressive totalitarian regimes that were willing to use war to remake the international order.
The Treaty of Versailles created the conditions. Germany was humiliated but not permanently weakened, creating a nation with both the capacity and the motivation for revenge. The reparations burden and war guilt clause generated deep resentment that extremist politicians — above all Adolf Hitler — exploited relentlessly. When the Great Depression struck, it destroyed the fragile stability of the Weimar Republic and opened the door for Nazi power.
Hitler's foreign policy was systematic aggression. He remilitarized the Rhineland (1936), annexed Austria (the Anschluss, 1938), demanded and received the Sudetenland at the Munich Conference (1938), occupied the rest of Czechoslovakia (1939), and finally invaded Poland on September 1, 1939, triggering declarations of war from Britain and France. At each step, the Western democracies chose appeasement — hoping that satisfying Hitler's demands would prevent war. This policy, forever associated with British Prime Minister Neville Chamberlain's claim of 'peace for our time' after Munich, only emboldened Hitler to demand more.
The war had a separate Asian dimension. Japan, driven by imperial ambitions and resource scarcity, had invaded Manchuria in 1931 and launched a full-scale war against China in 1937. Japanese expansion threatened American, British, and Dutch colonial interests in the Pacific. When the United States imposed oil embargoes to pressure Japan, the Japanese military leadership concluded that war with America was inevitable and launched the surprise attack on Pearl Harbor on December 7, 1941.
The failure of collective security was fundamental. The League of Nations, created to prevent exactly this kind of aggression, proved impotent — it had no enforcement mechanism and lacked American membership. The Western democracies, scarred by World War I and preoccupied by the Depression, were unwilling to confront aggression until it was too late. By the time they acted, they faced a coalition of militarized, ideologically driven states that had already gained enormous strategic advantages.