I think the right thing to do is get world support. Despite what some think, the US should not be a bully nation, nor should we foot the bill for everything which happens in the rest of the world. A coalition means shared expense, as well.
There's no clear evidence such weapons were used at this point. Were we to get involved, it should not consist of backing any particular armed faction. None of them are people we'd particularly want to be running a country--they are all Islamists of one flavor or another. At most, we could help stem the humanitarian crisis, but we have a very poor track record of choosing sides to pick winners and losers. And getting involved in other people's civil wars is just a bad idea in general.
No. What about Great Britain, Spain, Italy, and Japan, for starters? Though perhaps not themselves a nation at the time, what about the Russian Bolsheviks? In addition, what about Agent Orange? Or, are you arbitrarily restricting the list to nations that explicitly targeted people?
It seems from reports that both the rebels and the Syrian government are using chemical weapons. The best option for the US would be to do nothing and let this civil war continue until both sides want a ceasefire and then call for international assistance.
One observation I would make in reading and writing about this on a regular basis is the coverage about the new intelligence, and our politicians reactions to it, resembles the media drumbeat that proceeded the Iraq invasion. Chemical weapons notwithstanding, it's no wonder the US or other foreign powers seem uninterested in doing anything. Some of the rebel fighting groups like Al-Nusra Front are linked to al-Qaeda, with several others in the realm of islamic extremism. Even if the makeup of the rebel forces was not a complete clusterfuck, there aren't any consistently held rebel areas to stage forces and supplies like their were in Libya
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.