You may be confused by the sudden ratcheting up of the rhetoric on Syria, now that national leaders and opinion formers are back from their holidays. You wouldn’t be alone.
Clearly, the use or otherwise of poison gas in the conflict has little do do with it: there’s no logical or legal “red line” here, even if it turns out that (1) this was a deliberate CW attack and (2) the government was responsible, neither of which is clear at the moment, and neither of which, indeed, may ever be known for sure.
The explanation, as so often in politics, is more easily found by turning the problem round and looking at it from the other way. The decision to intervene militarily was probably taken several weeks ago, and what was needed was a pretext, given the very limited enthusiasm for another war in the Middle East in most western countries. This is that.
But what would an attack hope to achieve? Western leaders are not stupid, and they are hardly likely to send aircraft to bomb targets in Syria in the face of sophisticated air defence systems which might destroy some of them. The most likely scenario is a limited attack by air and sea-launched cruise missiles, aimed at very precise targets: probably Assad, his family and his key supporters. Why?
It’s only speculation, but I can imagine something like the following argument being made. The war shows no sign of ending soon, and the longer it goes on, the more damaging it will be to the region and the more radicalised each side will become. That being so, there has to be a solution of some kind soon. If Assad could be killed, or even receive such a shock that he fled, and handed over power, then maybe a government of “moderates” of different parties could emerge, and the situation could be rescued. This isn’t a stupid ambition, but we might learn very quickly whether it’s a realistic one or not. I have my doubts, but when you back yourself into a corner as the West has done, wrongly believing that Assad would fall quickly, there probably isn’t that much of an alternative.