Oh, that's an easy one. Those 50k troops sensible people are determined to leave behind as a residual force are going to have a very clear-cut mission. First, they're going to march inside all the bases we've established and lock the gates behind them. Then, they're going to sit there.
I mean, sure, they're going to have to shoot any and all of those awful brown people who dare to venture near our oasises of American civility as the country around them burns to cinders. And I imagine they'll need to give out ritual backslaps to the pilots who've finished bombing the hell out of anybody who's gotten their head held too high. And, maybe, if they're lucky they'll get to keep naked dogpiles of insurgents in the basement.
Those troops are nothing more They're a sop to the idea that America just doesn't lose wars. A way we can still swagger around in manly ways and wag our military industrial complex around.
But their real role is just to stay there and shoot nasty looks at people. Act as a speed bump if say, Iran or China from pouring over the border. And to give us an free standing excuse to re-invade when and if things go south, again. The operative comparison here is Korea, after all. Didn't anyone else get the memo?