Europe is not allowed to have its own foreign policy, let alone "side with China." All of those US military bases are there for a reason.
The whole point of them was to give the US influence while improving US security. Given Europe can't trust Trump will come to their aid, they won't give the US as much influence over Europe.
Here's a hint! It's your military. To put it bluntly, European nations and other US allies pretend the dollar has actual value and the US in turn guarantees security and backs the world order based on the rule of law.
Looks like the US is looking to pull out of its end of the deal. That's fair enough, being the world's policeman is sure a heavy burden to carry. I just don't see many people recognizing the implications for the US economy.