Asklemmy

It really depends on how far back you want to look.

If the US was to suddenly stop projecting its interests internationally, then as others have mentioned, then likely the world work become somewhat more socialized. European countries would probably step up and try to keep China in check, but without the US contributing to these efforts, it would cause a significant strain on their military resources.

If the US was to take an isolationist policy 100 years ago, then there is a good chance that WW2 would have been won by the Axis. The Allied forces likely would have put up a good fight, but I'm not sure they would have emerged victorious against the combined Axis forces. The war in the Pacific would have raged on much longer, and without nuclear weapons, there would have been an extreme loss of life invading Japan. At the very least, WW2 would have lasted much much longer than it did. Depending on the outcome, plenty of countries might currently be speaking German and debating if they should tear down 80-year-old statues of Hitler.