I don't see this happening as far as U.S. troops taking over another nation, to start bar a nuclear war and even if there was a nuclear war humans won't just all spontaneously go back to the stone age, I think we have reached the point where tech just isn't going to immediately be forgotten about there are too many literates in the world and too much books an information to lead to that much of a societal breakdown.
What is more likely is a sharp decline of economic growth caused by a stock market crash and debt crisis mixed with several other issues emerging at once which would drag down the world economy and geopolitical order as well as immense stagnation of society bordering on regression.
In such a case however, I doubt the U.S. Military remnants will get into the business of taking over countries I mean we could barely conquer Iraq, never mind a European country even if severely crippled and never mind an African nation which due to their lack of industrialization means they would be reeling even less due to a worldwide economic shock.
Even if the nation U.S. Troops were station in was as small as say Luxemburg with no army other regional powers are likely to intervene against the aggression if only to distract their people from other problems.
The final nail in the coffin is that American Soldiers aren't fighting for their officers they are fighting for America and their families back home and they are going to be worried about them over any idea of conquest of some random country in the Middle East, Africa, Europe of Asia, they will probably be abandoning their post in mass and trying to convince every captain in the Atlantic or Pacific to get them home.
In fact, the worse I could see a U.S. Military unit getting into chiefly would be to do what the
Czechoslovakian Legion did in the Russian Civil War and march their way through the chaos of whatever poor country they happen to be stationed in encountering armed resistance all the while, before forcibly acquiring a ship at the nearest port regardless of whatever that countries remaining official mays think on the matter, just so they can get home ASAP and get a sitrep on the situation before trying to help piece things back together, while all the while getting into many misadventures along the way.
