How did FDR win a "back door victory"?

FDR had long made his views clear about Hitler's ideology and take-overs in Europe. He could not send our troops there because no act of aggression had happened to the US. The US people did not want to go to war as they had just survived WW I and the Great Depression. They wanted to practice the idea of isolationism and take care of themselves. However, with the events of Pearl Harbor, the people had been attacked on US soil which gave FDR the chance to ask for a declaration of war. This was granted and FDR was able to send troops into Europe and the Pacific Theater. This was the "backdoor" that got us into the war. It was evident FDR wanted the European front solved first because he left the Philippines without proper weapons, food and military products then withdrew MacArthur who was supposed to go back to rescue those left in Bataan. FDR said they had to fight Hitler first and it was 4 years before what was left of the soldiers and civilians were rescued from the Japanese prison camps.