Feel free to disagree if I’m wrong. In the excellent BBC documentary “the world at war” from the 1970s, it was said that a month or two after Pearl Harbor (maybe Feb 1942?) FDR had a meeting with the top military planners in the US (army, navy, civilian). At that meeting it was decided that the United States should direct 90% of its military war effort against Germany, Japan was thought of a laughable enemy, incapable of launching any long term offensive.
Japan is resource poor, it is a mountainous country with very little oil. Also they must import food, so they could very easily be cut off from their supplies.
Germany on the other hand was viewed as an unstoppable enemy, having conquered so much of Europe in so little time. They had great military geniuses (Guderian, Rommel, Manstein, von Rundest and many others)
Even though the Russians were doing the heavy lifting on the eastern front, it was still felt that the US must devote 90% of its war effort against Germany, Japan was an afterthought)