OK, i know this is a fantasy, but what if the US did nothing? And said "we dont want to do anything in the future either until everyone (and that means EVERYONE on the UNSC if nowhere else) gets on their knees (really, literally on their knees) and begs us to please come and sort out the place because no one else can".
Forget about the Syrian people (whose sufferings will be legendary), what harm will come to the US?
Not a rhetorical question. I am curious to know what the harm of "inaction" and "weak leadership" is supposed to be?