I think you overestimate the cohesion of the USA. If we go to war with Russia there will be many segments inside the USA who view that as an opportunity.
The war in Syria is a good example. There are several sides in that conflict, each with their own agenda. The initial attack on Assad by fed-up citizens created an opeing for the Islamists. Suddenly Syria was not nearly as able to counter them as it had been.
To some small extent this happened in the USA during the Vietnam war. Some far-left factions tried to attack the government and open a second front in the USA. And, they were more successful than we acknowledge.
The widespread race riots resulted in a fundamental re-ordering of our society. And tribute is still paid to the rioters till this day.
The far-left's other ploys accomplished a lot less. But still there is a thread there connecting the revolutionaries of the 1960s with the leadership of today. (And not just in the USA, but even more so in France, Germany and other countries).
This man was lauded in a feature article in the New York Times the week Sept. 11 happened. This man is a confidant of our current President, and likely wrote his "autobiography".
A picture is worth a thousand words. This picture ran in the Sunday New York Times magazine, possibly the very highest status periodical among the American elite.
It's message is the same as a picture of Castro with his feet up on the former President's desk, smoking a cigar. "We won. We didn't compromise. We won."