I think the core problem is that military adventurism in this region shouldn't be predicated solely on, say, chemical weapons. Realistically chemical weapons don't tip the balance scales very much in the realm of America's narrow/selfish geopolitical interests, in the interests of humaneness, or in the more broad interests of attempting to do whatever is best for the people of the region on a long-term basis (specifically in regards to peaceful co-existence and consensual governance).
We've attempted low touch "tomahawk diplomacy" before. We bombed Saddam's Iraq for their intransigence and aggression in the late '90s, we bombed Afghanistan and the Suddan in 1998 in retaliation for the bombings of our embassies, and so on. For the most part such things are utter wastes of effort.
Retaliatory strikes are almost always bullshit. "Proportionate response" is just a fancy word for retaliation or revenge. Low touch warfare is almost always a mistake (see also: drone campaigns). We need to have clear geopolitical objectives, we need to be even clearer how we plan to achieve those objectives, and we need to follow through with as much effort (in whatever form) is required to achieve those objectives. Anything else is like some sort of macabre lottery. Attempting to see if killing a few people will magically result in a desired outcome even though the chances are low.
Granted, one should be under no illusions, there are some very serious "bad guys" in the region, and in Syria specifically. Bad guys who few people on Earth should object to being killed. However, the situation is also much more complex than that and it's never the case that military action only kills or injures the exact people you want and no one else. There is a 3 (ish) side sectarian war in progress in Syria which has now spread to Lebanon. Taking out the al-Assad regime could perhaps be a good thing but it won't bring an end to the sectarian war. Whether or not chemical weapons are used as long as that war continues tens of thousands of people are going to be killed each year it goes on, if not more. I don't think the Obama administration has a very strong understanding of the complex dynamics on the ground in Syria nor do they have a firm plan on how to end the war there. Moreover, I think the lessons the administration has taken from what happened in Libya (even taking into account the later attack on the US embassy) are likely to lead them to believe that the situation is far less complex than it actually is.