The most important decision FDR made during the war was one of his first.
After Pearl Harbor, American anger was overwhelmingly directed at Japan. Yet Roosevelt knew Germany posed a greater threat. Britain and the Soviet Union were under intense pressure from German forces and FDR could not risk their collapse. Well before the United States entered the war, he approved a strategy of defeating "Germany First," while maintaining a holding action against Japan. He stuck to it despite opposition from many Americans and some of his own military commanders. For example, he insisted upon a 1942 invasion of North Africa over their objection. The invasion achieved FDR's goal of quickly putting America's military into action against German forces to focus attention on Europe.