I recently read the book 1945 by Newt Gingrich and Harry Turtledove, where the premise is that Hitler got injured, and his advisers chose to wait to attack Russia until winning on the western front, meanwhile the US fought and defeated Japan without ever getting involved in the war in Europe (due to various political issues that are mentioned in the book). Because of this, the Nazis won the war, and now control almost all of Europe, excepting Britain. What I'm asking is are there any indicators of what would've happened without US involvement (which was far from guaranteed in the first place)?
What were the actual U.S. contributions to the Allies in World War II, and how did they compare with the hypothesized U.S. contributions in the 1945 book that featured a the Nazis getting a "Cold War" (rather than total domination) against the United States. Note that I'm not asking for speculation, rather for historical facts of any kind that might shed light on the subject.