If the United States got directly involved in the Syrian civil war, Bashar al-Assad's regime would eventually fall. He would surrender or die eventually. However, this is so unlikely to happen that it hardly bears discussion. Bashar al-Assad would surrender if an advanced race of space beings descended on Syria and fought on the side of the rebels. That scenario is just as likely to occur as the United States getting directly involved in the conflict.
Yes, I believe that if Barack Obama wanted to get involved in that situation directly, with troops, there would be nothing that al-Assad could do. The military of the United States is just do mighty and there's not a country that's even close to its level. He would have to surrender when it comes down to it.
I believe Bashar al-Assad would surrender if the United States military was directly involved in Syria's civil war, but I have a problem with this suggestion. I do not believe the United States has any business interfering with Syria's Civil War and this is exactly why we haven't done anything thus far. We don't belong there.
No, I do not think that Bashar al-Assad will surrender if the United States military gets directly involved in Syria's civil war, because he has come too far to back down. Al-Assad thinks that he is the legitimate leader. He knows that he will be hanged if he surrenders, so he cannot back down.