The Second World War (1939–1945) was a conflagration without precedent. The war left millions dead, mortally weakened the colonial empires, and gave birth to a twilight struggle between competing ideological systems. It was, in sum, the defining event of the twentieth century. Some aspects of the war to consider as you read:

• Appeasement. The causes of the war are complex but must include the failure of western democracies to take seriously the threat of fascism. When Japan invaded Manchuria in 1931, when Italy seized Ethiopia in 1935, and when Germany claimed the Sudetenland in 1938, world leaders opted for appeasement rather than risk a war.
• Total war. Like the First World War, the Second World War involved whole populations on an unprecedented scale. Women on both sides performed industrial work and joined auxiliary forces. Civilians were targets of war through aerial attacks, blockades, rape, and internment. Civilian casualties were in the tens of millions.
• Genocide. The most horrifying aspect of the war was the Nazi attempt to methodically exterminate the entire Jewish population of Europe, along with other "undesirable" populations. Nearly six million Jews were killed in the death camps.
• An uneasy alliance. Capitalist and communist states found common cause in the battle against fascism. By keeping up the pressure on two fronts, the Allies eventually crushed Nazi Germany and its allies. However, by the end of the war, the alliance between Britain, the United States, and the Soviet Union was frayed and unstable.
• Postwar uncertainties. By the end of World War II, the difference between the United States and the USSR were increasingly apparent. The two states managed to paper over their divergent perspectives and goals at the Yalta and Potsdam Conferences in 1945, but, by 1947, Europe was divided into an American-dominated liberal-capitalist west and a Soviet-controlled communist east.