Could the US take out ISIS in 6 months? Probably.
But look at the last time we created a power vacuum in the region, what did it lead to? Oh, that’s right, ISIS.
The only time the US was at all successful in nation building was the Phillipines, and that took holding on to them for about half a century. That’s the sort of commitment we’d be looking at if we were to just go in and smash ISIS and hope for anything resembling a realistic chance of it working out well.
And all the other countries in the region might get nervous about their land borders with a long term US occupation, which wasn’t an issue in the Phillipines. And the Phillipines was taken after a war more easily digested as legitimate both here and abroad, reducing the global outcry over our occupation. The situation would be much more dangerous for a long term occupation in the middle east.
This is one where we really need to leave the bulk of the solution to local powers. It’s conceivable that it would be a good idea to put more boots on the ground to help, but local powers really need to take the lead on sorting this out. Things don’t go well when we come in and impose solutions.