George Friedman of Stratfor has an essay entitled Obama’s Bluff that has what I think is the best analysis of the problems the USG faces if Syria has crossed the President's red line. The whole thing is worth a read, but here is a flavor:
The question therefore becomes what the United States and the new coalition of the willing will do if the red line has been crossed. The fantasy is that a series of airstrikes, destroying only chemical weapons, will be so perfectly executed that no one will be killed except those who deserve to die. But it is hard to distinguish a man's soul from 10,000 feet. There will be deaths, and the United States will be blamed for them.
The military dimension is hard to define because the mission is unclear. Logically, the goal should be the destruction of the chemical weapons and their deployment systems. This is reasonable, but the problem is determining the locations where all of the chemicals are stored. I would assume that most are underground, which poses a huge intelligence problem. If we assume that perfect intelligence is available and that decision-makers trust this intelligence, hitting buried targets is quite difficult. There is talk of a clean cruise missile strike. But it is not clear whether these carry enough explosives to penetrate even minimally hardened targets. Aircraft carry more substantial munitions, and it is possible for strategic bombers to stand off and strike the targets.
Even so, battle damage assessments are hard. How do you know that you have destroyed the chemicals -- that they were actually there and you destroyed the facility containing them? Moreover, there are lots of facilities and many will be close to civilian targets and many munitions will go astray. The attacks could prove deadlier than the chemicals did. And finally, attacking means al Assad loses all incentive to hold back on using chemical weapons. If he is paying the price of using them, he may as well use them. The gloves will come off on both sides as al Assad seeks to use his chemical weapons before they are destroyed.
A war on chemical weapons has a built-in insanity to it. The problem is not chemical weapons, which probably can't be eradicated from the air. The problem under the definition of this war would be the existence of a regime that uses chemical weapons. It is hard to imagine how an attack on chemical weapons can avoid an attack on the regime -- and regimes are not destroyed from the air. Doing so requires troops. Moreover, regimes that are destroyed must be replaced, and one cannot assume that the regime that succeeds al Assad will be grateful to those who deposed him. One must only recall the Shia in Iraq who celebrated Saddam's fall and then armed to fight the Americans.