No he did not.
They used Chlorine and that is not on the banned list.
They use these chlorine bombs ALL the time with no US response.
This is a change in policy. Plain and simple.
They said he used chlorine, but they aren’t sure if anything else was used.
Not a change in policy really. There was no policy on chlorine. It was a casus belli that could be used at any time.
Trump wants out and thinks he can do it now.