You'll definitely lose a lot of influence, the sort that comes from leading the free world.
I'm hoping you don't become the new Russia, which looks a real threat at the moment.
There are still a lot of countries that believe in freedom and the rule of law, collective security, and so on. So we'll continue with that.
If push comes to shove, Europe ought to be able to defend itself against Putin without American support, especially given the Russian army's massive losses.
They have nuclear weapons, but they'd be nuts to use them, because we have them too, even if not as many. So it will probably be a conventional war.