Gassing noncombatants is obviously a terrible thing, but to make a distinction between conventional and nonconventional weapons strikes me as arbitrary. The kind of shelling that was going on in Aleppo is just as destructive as the use of a gas, so it's not clear to me why you draw the red line here. It's kind of left over from WWI and the use of gas there and the 1925 treaty.

http://www.theatlantic.com/national/archive/2013/09/what-makes-america-so-prone-to-intervention/279393/