When you're in the middle of a hard fought war you do not have the certainty afforded by history. Everything is at stake. You may think you're ahead but then

When you're in the middle of a hard fought war you do not have the certainty afforded by history. Everything is at stake. You may think you're ahead but then you suffer unthinkable losses (the battle of the bulge, for example --for both sides and the Korean war). With large scale wars where both sides both have a chance at winning, it's not over till it's over. You have no idea what will happen you cannot afford to say, "I think we'll win, let's pull back". Your fleet can be sunk, your ally can fall unpredictably (leadership death), etc. You also have to remember, the Axis Japanese were extremely brutal in war. The allies could not afford to think, "maybe they'll turn nice".