He has used nerve gas before, on a smaller scale.
The response of the West was exactly what you are advocating we do again.
Nothing.
The result was a bigger atrocity next time.
Why should we expect a different result from the same response.

A few hundred died in the latest atrocity.
Would it make a difference if it was a few thousand next time.
A large attack on a city like Homs would easily achieve that.