I think Taking it might be possible in Stalingrad sense but they arent going to hold it for more than few weeks before its again contested.
If it's Damascus, I am not sure. The Syrian regime had basically no secure base to launch a (quick) reconquest if Damascus falls in 2015 and might collapse. Not that a fall of Damascus to ISIS was ever particularly likely (though probably marginally more so than Baghdad).
The results woul be... messy. Obama would be reluctant to go "boots on the ground" again, but it may be his least bad option. Detente with Tehran would become a
big priority. I think not a "fine if you have nukes" sort of priority, but still. The US would probably be OK with propping up all sorts of Syrian opposition groups, likely the same groups that Turkey and Qatar (on one side) and Saudi Arabia and the Emirates (on the other) tend to like, with some partiality to the former.
Regional alignments going full circle, but a non-Euclidean circle (that is, vaguely US-aligned Iran, Turkey, Iraq and Qatar vs. Egypt-Saudis-Israel leaning Russia) is theoretically the way this goes if allowed to go on long enough (not going to be fully happening 2019 anyway) but I am sure that there would be plenty other factors preventing this engame.