World War II Begins

World War II began in Europe on 1 September 1939, when Germany invaded Poland, and ended in 1945 after becoming the deadliest conflict in human history. Britain and France declared war on Germany two days later, turning a regional attack into a general European war that would eventually spread across Asia, Africa, and the oceans. Estimates of the dead vary, but most historians put the total at well over 60 million people, a majority of them civilians.

The roots of the war lay in the aftermath of World War I. Many Germans resented the Treaty of Versailles, and the economic devastation of the Great Depression helped bring Adolf Hitler and the Nazi Party to power in 1933. Hitler pursued rearmament and territorial expansion, openly justified by racial ideology and the idea of winning "living space" in the east. Germany reoccupied the Rhineland in 1936, annexed Austria in 1938, and took the Sudetenland that same year after the Munich Agreement, in which Britain and France hoped concessions would preserve peace. Germany then occupied the rest of Czechoslovakia in March 1939, undermining that hope.

Shortly before the invasion, Germany and the Soviet Union signed a non-aggression pact, which included a secret protocol dividing Eastern Europe into spheres of influence. Germany attacked Poland from the west, and Soviet forces entered from the east in mid-September. Poland was overrun within weeks. Britain and France offered little direct military help at first, and the following months in the West became known as the "Phoney War." In 1940 Germany conquered Denmark, Norway, the Low Countries, and France, leaving Britain to fight on alone.

The conflict widened dramatically. Germany invaded the Soviet Union in June 1941, and Japan, already at war with China since 1937, attacked Pearl Harbor and European colonies in Asia in December 1941, bringing the United States into the war. The Allies, chiefly the United States, the Soviet Union, and Britain, gradually gained the upper hand. Germany surrendered in May 1945, and Japan followed in August and September, after the atomic bombings of Hiroshima and Nagasaki and the Soviet entry into the war against Japan.

The war's consequences reshaped the world. The Holocaust, in which Nazi Germany and its collaborators murdered about six million Jews along with millions of others, became a defining symbol of human cruelty and led to new ideas about genocide and human rights. Europe lay in ruins and was divided between Soviet and Western spheres, setting the stage for the Cold War. The United Nations was founded in 1945, European empires began to lose their colonies, and the United States and Soviet Union emerged as the dominant powers.