Answer:
U.S. entry into World War II was the deciding factor in the success of the Allies. Prior to joining the war, the United States maintained a quasi-isolationist approach. The Japanese attack at Pearl Harbor in 1941 proved decisive in the United States joining the war. Both France and England were suffering heavy losses in the face of the German war machine. France would eventually fall and be occupied by the Germans. A collaborator government, the Vichy was established there. Smaller European countries were invaded and occupied by the Germans. Britain endured the German blitzkrieg, which decimated its major cities. American land and naval forces brought much needed relief for European forces.
Explanation: U.S. Entry into World WAr II was a result of world geopolitics and the close alliances between America and Europe.