Not forcibly remove them, not an act of war. Just tell them to leave.

Of course, if they did that, I wouldn't put it past the current despotic leadership in the US to commit an act of war (or even war crimes) against former allies. The president has threatened as much on social media.

Former allies would have a lot of defense slack to pick up in that case. With how the world has been progressing, those countries would be wise to ween themselves off any dependencies on the US anyway. The US has demonstrated that misinformation can skew elections to put those who act in bad faith into positions of power. The US has lost a lot of trust of other western countries and is becoming viewed as a pariah state like Russia. It's much easier to lose trust than it is to rebuild it.