Sept. 1 may be the official start of World War II, but it didn't start in a vacuum. Europe and Asia had been tense for years prior to 1939 because of the rise of Adolf Hitler and the Third Reich in Germany, the Spanish Civil War, the Japanese invasion of China, the German annexation of Austria, and the imprisonment of thousands of Jews in concentration camps. After Germany's occupation of areas of Czechoslovakia not previously agreed to in the Munich Pact and its invasion of Poland, the rest of Europe realized it couldn't try to appease Germany any longer. The United States tried to remain neutral, and the Soviet Union invaded Finland.

1939

The first full year of the war saw Germany invading its European neighbours: Belgium, the Netherlands, France, Denmark, Norway, Luxembourg, and Romania, and the bombing of Britain lasted for months. The Royal Air Force undertook night-time raids in Germany in response. Germany, Italy, and Japan signed a joint military and economic agreement, and Italy invaded Egypt, which was controlled by the British, Albania, and Greece. The United States shifted to a stance of "nonbelligerency" rather than neutrality so it could find ways to help the Allies, and the Lend-Lease Act (the exchange of materiel aid then for 99-year leases on property to be used for foreign military bases) was proposed late in the year. Popular opinion still didn't want Americans in another war "over there." The Soviet Union, meanwhile, took part of Romania and installed Communists in the Baltic States, later annexing them.

1940

The year 1941 was one of escalation around the world. Italy may have been defeated in Greece, but that didn't mean that Germany wouldn't take the country. Then it was on to Yugoslavia and Russia. Germany broke its pact with the Soviet Union and invaded there, but the winter and Soviet counterattack killed many German troops. The Soviets next joined the Allies. Within a week of the Pearl Harbour attack, Japan had invaded Burma, Hong Kong (then under British control), and the Philippines, and the United States was officially in the conflict.

1941

U.S. troops first arrived in Britain in January 1942. Also that year, Japan captured Singapore, which was Britain's last location in the Pacific, as well as islands such as Borneo and Sumatra. By the middle of the year, though, the Allies started gaining ground, with the Battle of Midway being the turning point there. Germany captured Libya, but the Allies started making gains in Africa, and Soviet counterattacks made progress as well in Stalingrad.

1942