If UK and Russia was captured before US sent many troops to Europe, I think the US would not go to war against Germany, only defend their home waters, and eventually secure south America, and capture Japan.
For Germany’s sake, this had to happen before, or sometime during 1942, b/c then the war was lost for Germany.
It is ofc very possible that nukes against Germany could be an option, but even today, you need troops on the ground, or at least be 99% certain that your enemy knows that you’re willing to put troops on the ground, like the 1999 NATO bombing of Yugoslavia.
It wasn’t until Milosevic thought that NATO would send in ground troops that he conceded, even if he (Serbia) had lost before the war started.