France over-run by Germany. Gestapo at work, with all its horrors.
UK+US land in French North Africa, part of taking Africa from Germany, part in the long run of liberating France from horrors of occupation.
French soldiers fighting, killing and being killed by UK+US troops.
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.
My thought here was that armies can fight with the most brutal oppressors of their very own country, against those who would liberate it.
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.