Couldn't they put a couple million men on a battle field on 2 fronts?
Syria's military is a joke. Let's consider that they tried to attack us, one carrier battle group could decimate their entire infrastructure in both military and civilian terms. Take out their power stations, their roads, defense installations, government buildings, airfields and fuel depots and they are screwed. And that would take all of about 48 hours. Since we would control the air, their troops would be sitting ducks.
Iran could try and invade from the west but we would destroy their gasoline refineries and no army travels far without petrol. We would do the same things to their defense infrastructure as well. Their air force is flying old planes and their pilots do not compare with ours. They have a limited navy that would be sunk in no time and their armor is no match for ours. Our soldiers would take casualties but nothing on the scale that theirs would. Iran's population would cry "uncle" and perhaps even overthrow the government which would be fine with me.