The U.S spends more on military than the next 7 countries combined. It’d put up a good fight. Probably conquer Canada/Mexico in hours (I’d assume they would just concede the same way Paris/France conceded to the Nazis in WW2), and then we’d also have the advantage in Eurasia. We could nuke most of western Europe, and the only country that could really stop us is Russia because they also have a comparable number of nukes. If we successfully disarm them and are the sole nuclear super power in the world, I could see the U.S winning ww3 and becoming a global government.