You said that the whole World War was immoral, meaning that fighting against the Nazi's totalarian rule was immoral. I think that answers your question.
As for what if, have a look at what the Nazis had accomplished before we started fighting back. England was about the only free country in Europe. The Nazis had invaded Africa. Their allies Imperial Japan were taking over the Pacific Rim. You can rest assured that if we hadn't thought there would have been no Vietnam, no 9/11, because there would have been no Vietnamese or Muslims in the world, they would be under the heel of Nazi rule. Maybe that's what you want, but about six billion people in the world don't want that, not then and with the knowledge of what atrocities Hitler was behind certainly not now.