I would think we are still a long way off from doing anything. Kofi Anan has already said there is no military solution in Syria and despite their insistence that the use of WMD would constitute a 'red line' even Obama is being massively cautious and making oblique references to the Iraq debacle, after which it will take a hell of a lot more than the intelligence community declaring because we said so. That said, there has always been a conflict between the Spooks and the politicians and the later's use of what could be very nuanced intelligence reporting for blunt political purposes, so any argument between the Spooks and politicians could well degenerate into he said she said.
IMHO, the context of any alleged use rather than the actual use will be what acts as a trigger. If it can be proven that the Regime has ordered a strike, that is one thing. However, proving that conclusively will be very difficult. It could well be a case of a rogue commander at a local level making a decision on his own initiative. Or it could be a case of Syrian logistics not being up to par, with mixed stockpiles, poor record keeping, old weapons with markings that have become obscured all contributing to an inadvertent firing of CW rounds when Ahmed was told to break open a box, take said round and fire in a given direction.
Until you can prove the first of those without any doubt and can conclusively discount the other 2 contexts, then nobody in their right minds would get involved in a civil war on the grounds of alleged WMD use.