Whether it could have survived even with a US-Russian accord is open to doubt, given the incentives for al-Qaeda and its allies to destroy it. But the politics of the US-Russian relationship played a central role in the denouement of the second ceasefire agreement.

The final blow apparently came from the Russian-Syrian side, but what provoked the decision to end the ceasefire was the first ever US strike against Syrian government forces on 17 September.

That convinced the Russians that the US Pentagon had no intention of implementing the main element of the deal that was most important to the Putin government: a joint US-Russian air campaign against the Islamic State (IS) militant group and al-Qaeda through a "Joint Implementation Center". And it is entirely credible that it was meant to do precisely that.