The question is what you mean by "win". It would not be very reasonable to describe reality with the statement that the US lost a war against some group or groups in Iraq. But they didn't wipe those people out, and consequently those people can still do things. The US would win in Syria if they did the same thing, and in the same sense, and with the same sort of consequences.

Tacitus said that the Romans were accustomed to "make a desert and call it peace." If the US wanted to win a war in that sense, they could. But they don't want to, and won't, basically because pretty much everyone considers it to be immoral. But as long as you don't do that, there will still be people there with the same ideas and intentions, and some of those people will act on those ideas and intentions.