Bedford Forrest, I had people on both sides, is one of my heroes, but what compels people to enlist in a foreign adventure.
Some serve in the Foreign Legion because they're on the run from the law and others because they are on the run from themselves. Some serve in foreign wars as mercenaries out of greed or because they know nothing else.
Some are compelled to serve in foreign adventures because, quite simply, they are forced to do so by the draft. Others out of poverty.
But it is not the motivation that gets them there which is intriguing, it is what compels them to behave the way they do once they are there.