There's a concept that I don't quite remember the name of, but basically, if some American troops are stationed somewhere permanently, that means that if you invade that place, you are attacking America. The troops have some value outside this, but they are also a sacrifice to induce a response.
Do you mean tripwire force?
The idea behind them, using your example, is that when the invasion happens, it will kill a number of American troops, whose death will act as a tripwire to triggering full size war.
Its one of the methods for deterring invasions.
