If you agree about iraq, it should be fairly easy to see the the similarities with Syria. The main difference being the will of the American people to fight another war like that. Our government wasnt naive to the fact that American citizens weren't going to allow another illegal invasion of a sovereign country. Syria was a legitimate state ran by a legitimate govt/ruler, albeit not pro-US. So, we cause unrest by aiding opposition. We demonize and flat out lie to make our interests and agenda appear legit, continuing to this day. Our govt has proven time and time again that it will say and do anything, and when all else fails, invade illegally. It's the same thing now in syria.
A legitimate government is decided by the people. There is opposition in every country in the world, including here. If the people who opposed the wrong doings of our govt decided to protest en masse, it would look like a revolution. Without going too in depth, legitimacy of a government revolves around its moral standards and laws. The acceptance or refusal by the people of that state deem it legitimate or not imo.