This same scenario was in the "What Ifs of American History" book. The author thought that we'd still get involved later on. His scenario was that Japan would carefully avoid targeting American targets, concentrating on British, Dutch, and French possessions (would this give them the oil they need?). The US gets involved by sending supplies to various opponents of the Japanese, and some supply ships get attacked by Japanese planes. The US gets involved, although less enthusiastically (albeit there is not much resistance to the war), Germany still declares war on the US and vice versa, and the war comes to an end in '46, IIRC...