Fighting the Nazis was one thing. The Yanks had the will to keep fighting until the bitter end there. It was a just war, and the Yanks got involved when the Nazis stupidly torpedoed the USS Texas right off of New York City...
Unlike the Pacific War, which the US stupidly started. "MacArthur's War" they called it. That idiot got caught running guns to the Chinese, and the Japs had every right to seize those ships. And MacArthur was stupid enough to try blowing them up, and ended up sinking half a dozen Japanese warships. Nobody wanted to die bailing that idiot out. He got what was coming to him, too. His blunder cost the US dearly. Independence for the Philippines. Philippines permanently allied to Japan. Japan gains all of the other US possessions in the region. Japan gains the Hawaiian islands as a protectorate. The US can't keep any battleships or aircraft carriers in the Pacific ocean without prior consent of the Japanese. Total size of the US Navy limited to one and one half that of Japan, with all heavy units in the Atlantic. Panama Canal transfered to Panamanian control. All US trade with China must be sanctioned by Japan.
The US had the resources to easily win both wars at the same time. The nation was/is just that powerful. But in the Pacific, they didn't have the will to fight, even where they massively outnumbered the Japanese. In the end, it was easier for the Yanks to just swallow their pride and accept the humiliaton. All of their wrath, all of their might, was then focused upon the Nazis. The end of the Second Great War in June of 1944, and the fifty year Allied occupation of Germany shows just what happens when the yanks have the will to fight. The Pacific War serves to remind everyone just how soft they are if that will is... lacking.