Yes, it obviously matters what we're allowed to change. Quickest I can think of is better intelligence has the American Pacific fleet ready to fight as the Kido Butai approaches Pearl Harbor, positioned to counter-attack as soon as the Japanese move. Naval battles of the era involved a lot of luck; have the breaks favor the U.S., and the destruction of much of the IJN's strength in that battle becomes possible. U.S. follows up by moving fleet to Manila, from which they can interfere with Japanese supply lines and use shore bombardment to harass the invasion forces; again, with some luck maybe the Philippines hold (ideally also scrape together some reinforcements as well). Such a large U.S. force in the area, and a heavily weakened IJN, would also greatly reduce the gains the Japanese could make against the Allies in the Indies. It still would take a while for the U.S. to ramp up for major offensive operations, but following Pauh's suggestion and having the U.S. ignore Europe means, for example, the forces used for Torch could be used against Japanese positions, so the Japanese could be losing ground quickly before 1942 ends, and I could see them besieged in the home islands in 1943. Hard to get more exact with the dates, as this would involve massive departures from OTL, of course. Also not clear at what point the hopelessness of their position would produce a surrender; they were pretty stubborn, and if we're trying to make it happen earlier nukes won't be available, but the fact that in this scenario the Japanese do not have a string of early successes to bolster the prestige of the military may make the militarists more vulnerable and less able to control policy. Without a surrender, an invasion of the home islands would involve a huge amount of preparation; again, since we're assuming the U.S. isn't involving itself in Europe, perhaps they might be ready around when they were ready for Normandy OTL. My thoughts, anyway.