It was the rebels that used gas, not Assad. Assad gave them up quickly after the accusation just for that reason.
The red line was an Obama scam.
My opinion? It doesn’t matter. It’s not our mess. Just help to arm the Kurds to fight ISIS.
Both sides were using them, and for quite a while before the press reported on it. Assad “gave them up” after Obama backed down but of course there is no way to verify that he did give them up.
The claim that it was the rebels that used gas was from Seymour Hersh, who is rather notorious.