Chemical weapons have been used extensively in the last century. WWI didn't even begin to put an end to that.
As for Syria, there's no reason for the US to unilaterally intervene. It's not like Afghanistan, where an ally was invaded, or Iraq, where the invader of an ally was in violation of their terms of surrender. Syria would be another Vietnam, and end just as badly.
If the motivation is the humanitarian disaster, it needs to be the United Nations that intervenes. I would not be opposed to being part of a multinational force, as long as it were planned and executed competently.