No I don't believe it was even said that we "wouldn't fire a shot" in Afghanistan.
Was it?
But this is a different scenario entirely. It would be madness to intervene militarily in Syria, which is after all experiencing a civil war. I don't believe for one moment that that is an option.
However, on the other hand there are considerations such an containment of the conflict, as Sir David says, policing of humanitarian zones, and even just putting the frighteners on Assad and co.
It would be foolish to have no contingency at all. Military assistance in the long run, say after the current regime has been ousted, might be feasible.
It would plainly all be hugely risky however.