The government of Syria does not want US troops in their country. Assad's troops may or may not try to fight them.
I could say that this invasion is wrong, but moral arguments have had zero effect on US foreign policy during this century. I do believe that this invasion unnecessarily puts troops in harm's way, and I want them all to come home safely.
I hope our troops are not attacked by Assad's forces because that will likely mean that our military will be forced to overthrow Assad. If Assad is smart and doesn't attack US troops, he is a lot more patient than most men you will ever meet.
To be sure, President Bashar Assad is a terrible human being. The ISIS thugs that oppose him are terrible people. Many of the non-ISIS rebels are horrible in completely different ways. But to forcibly remove Assad from power will likely mean that a different tyrant will be in charge, or nobody at all will be in charge. We can hope that Assad will eventually retire or die of natural causes, and power is peacefully transferred to sane, rational beings.
In the mean time, the most realpolitik thing to do is to let all these fanatics kill each other without our help. The most humanitarian thing to do is allow refugees to settle here. Many people seem to think that the latter option is too risky. While I might disagree with that, I would hope that they would agree that the first option is safer than what's actually happening.