World War II was a war fought between the Allied (United States, France, and the Soviet Union) and the Axis (Germany, Japan and Italy). The War began when Adolf Hitler invaded Poland. Hitler's Nazi aggression, the bombing of Britain, and Occupation of France called for the Allied to stop Hitler's campaign. The War was settled at the Yalta Conference where the Big Three (Stalin, Churchill, and Roosevelt) discussed Europe's post-war reorganization.
(McKay p. 922-938)