I think the story goes is that US planes actually observed the Syrians dropping barrel bombs with the sarin from planes, I'd have to dig up the story but read that somewhere.
Anyway, Assad, Iran's proxy and Iran killed US soldiers in Iraq, Assad allowed Jihadists to operate from Syria; Syria is a designated state sponsor of terrorism and has been on that list for over 20 years, maybe since '79, they have assassinated Christian heads in Lebanon, obviously weapons to Hezbollah and Hamas go through Syria and the list goes on. I'm for being cautious about war if I were to vote on it but I have little sympathy for Assad.