World War II began when Germany invaded Poland in 1939. The invasion began on September 1, but the war didn't begin officially until Great Britain and France declared war on Germany.
Prior to invading Poland, Germany took over Austria and Czechoslovakia. The nations of Europe, led by Great Britain and France, hoped that by allowing Adolf Hitler, Germany's dictator, to have additional territory, that they could avoid war. Hitler, however, saw the concessions as a sign of weakness and continued his conquest of Europe.
To create a justification for invading Poland, Heinrich Himmler devised a plot in which a prisoner was taken from a German concentration camp, dressed in a Polish army uniform and shot near the border between Germany and Poland. The Germans then used the dead soldier as evidence of a Polish attack on Germany. The German army quickly defeated the Polish forces and proceeded to attack and conquer most of Western Europe, including Belgium, the Netherlands, Norway and France.
The United States entered World War II on December 8, 1941 by declaring war on Japan for bombing the United States naval base at Pearl Harbor on December 7. The U.S. entered the war in Europe when Germany declared war on December 11, 1941.