World War II threatened Western values as no previous conflict had. After World War I, hopes for peace ran high. Diplomatic achievements such as the Washington Naval Conference and Locarno Pact, and the establishment of the League of Nations, seemed concrete evidence of the West's resolve never to permit another world war. However, postwar Europe proved incapable of dealing with a resurgent and resentful Germany. Once firmly in power, Hitler pursued an aggressive foreign policy driven by Nazi ideological aims. Internally divided and afraid of another war, France looked to Britain for leadership. Equally uncertain, Britain adopted a policy of appeasement. This policy enabled Hitler, between 1933 and 1938, to rearm Germany, remilitarize the Rhineland, complete the Anschluss with Austria, and, after the Munich Conference, dismember Czechoslovakia. Exacerbating the German threat to peace were Italy's invasion of Ethiopia, Japan's invasion of Manchuria, and the failure of France and Britain to oppose fascism during the Spanish Civil War.

Bolstered by pacts with Italy and Russia, Germany invaded Poland, which quickly fell to the blitzkrieg. In April 1940, Hitler began his conquest of western Europe, which concluded with the division of France into occupied territory and the collaborationist Vichy regime. After failing to win the aerial Battle of Britain, Hitler returned his attention to the east, invading the Balkans, Greece and, in April 1941, Russia. On their conquests, the Nazis imposed the New Order, using exploitation and terror to brutalize "inferior" subject peoples. Central to this order was the Final Solution through which the Germans systematically murdered about six million Jews. Resistance movements rose throughout Nazi-dominated territory, even in Germany itself.

With the attack on Pearl Harbor, both Japan and the U. S. entered the war. By the end of 1941 the Axis powers had reached the height of their success. However, crucial victories in 1942 at Midway, Stalingrad, and El Alamein turned the tide of the war in the Allies favor. After Midway, the U. S. gradually pushed Japanese forces out of the Pacific strongholds. After Stalingrad, Soviet troops won further decisive victories-e.g. Kursk-pushing the Germans back toward the Russian border. After El Alamein, the Allies invaded Italy, provoking the overthrow of Mussolini. On D-day the Allies invaded France. Caught between Russian armies in the east and allied forces in the west, Germany fought on into 1945, finally surrendering on May 7. Japan continued to resist, but after atomic bombs destroyed Hiroshima and Nagasaki, it sued for peace, surrendering on August 9.

World War II was the most destructive conflict in Western history. From it emerged a new world political order defined by super-power conflict, the dissolution of overseas empires, and greater cooperation in Europe. The brutality of the war, exemplified by the Holocaust, dealt another blow to Enlightenment values. However, most Western intellectuals responded by embracing those and contributing to the spread of liberalism and peace.