Not American, but provided US military has an oath towards the Constitution (and not to whatever the government claims), I doubt _all_ of US army would follow (either internally, either externally) such a brutal reversal of duty as well as alliances.
Duty is to the Constitution and the Commander in Chief. And alliances are at the discretion of the President. The military will do whatever they are told in terms of who the have to be friends with.
I know. That's the theory and mostly the practice.
Only, ask your military to return against your just previous allies (at your own initiative) among which the one that helped your very nation to fight for its independence, with which you did cross-training and exercises, for the past 80 years... everyone is in for quite a bumpy road.
You mean the _Vichy_ French soldiers? that's quite a different situation than the allied French army :)
And, I was more thinking of the situation on American ground, within the USA and between the USA and Canada. I don't mean it wouldn't happen. I mean that I don't think that would happen with 100% engagement from all US army. The disconnect and reversal of strategy of the US, against its own allies, is too sudden.
