1) In the 1930s, Adolf Hitler took actions that violated the Treaty of Versailles and increased Germany's military power, leading to Germany invading Poland in 1939 and starting World War II in Europe. 2) While maintaining neutrality at first, U.S. President Franklin Roosevelt grew concerned as Germany conquered more of Europe and began aiding Britain in 1940 through weapons sales. 3) The U.S. and Germany were on a collision course, but Japan attacked the U.S. naval fleet at Pearl Harbor in December 1941, bringing the U.S. into the war.