My father was in the occupation force in Japan after the war.  I remember him telling me back in the 1970s, that because of Japan's economic success, post war, some people said that THEY had actually "won" the war.  No, he told me plainly, they did not.  He saw the utter destruction and human suffering in Japan, including the cities that were hit by atomic bombs.  America also suffered greatly with the loss of over 400,000 men.  Think about the potential that was lost along with all of those souls.   To overlook all the pain, suffering, and destruction of the world war and to say it "did our nation a lot of good" from an economic perspective, seems like quite a stretch to me.  Regarding war with Germany vs. "Japan only," both nations were part of the tripartite pact and as such, war against one inevitably meant war with all three (Japan, Germany, Italy).