He does take care of one serious problem for the Japanese that is they are no longer required to keep most of their army committed vs the Japanese. However the mood in the US was turning more and more against the axis. Opinion polls that have been posted in other threads on this topic indicate that the US would probably have been in the war soon anyway. Indeed Roosevelt may well have been able to get a declaration of war in late 41 but without the level of support he wanted. Fresh off the diplomatic success of a settlement in the Chinese/Japanese war he might the US entry into the war is almost assured in 42. If this happens while the Japanese are oriented toward the Soviets they are going to have severe problems. Also once they start having significant problems the Chinese may join back in. The Reds will hardly have stopped in any case.