However, if any country used bio/chem weapons against us ... we would likely respond with nuclear weapons. This is because America does not have a working stockpile of bio/chem weapons. All the ones we have are outdated and unstable, and being destroyed.
Syria is a member of the UN and has had an ok relationship with US. The country is bordered by Lebanon, Turkey and Iraq the last one being a hostile territory I don't think that they will jeopardize their relation with us knowing that Iraq or Lebanon could at any moment launch an attack against.
I believe the response would involve the international community. It would be much more effective and better in the long term for the Syrians if done that way. We don't need to ride in there looking like we want to "conquer" them. If chemical weapons are deployed it would be in the best interest of Syria's neighbors to get this war stopped immediately.