After 40 or more meetings, no two foreign ministers can know each other better than the US secretary of state, John Kerry, and his Russian counterpart, Sergei Lavrov. Their latest marathon session in Geneva produced a new and very detailed plan for a truce in Syria and better coordinated operations, as a prelude to peace. The plan has the verbal support of the Damascus government. Could it work?

That the announcement was immediately followed by an upsurge in violence is no indicator. The delay between announcement and deadline invites all parties to maximise their advantage. The test is whether fighting indeed eases off by the designated moment: Monday at sundown.

The fact that the plan is underwritten by the US and Russia doesn’t guarantee success. Almost 25 years after the dissolution of the Soviet Union, a US-Russia deal about a third country has a distinctly retro feel. Syria’s civil conflict may have turned into a wider war, but there are many more players and proxies vying for the spoils than there were in any cold war contest. Washington and Moscow can no longer snap their fingers and command hostilities to stop.

Which is not to dismiss the deal. If the US and Russia can agree to cooperate in Syria, despite the coolness of their relations generally, then they both have a substantial investment in making it work. Some aspects of the deal – defining who they will fight and where – may be elementary, but this is progress compared with the absence of coordination before.

Also on the plus side are domestic considerations. This US administration has four months to run. President Obama came to office promising to end US involvement in foreign wars; he would surely like to leave the White House without the US, even tangentially, engaged in another war. Pilloried for not using force against President Assad three years ago, would he not seize a chance to show there could be another way?

Russia, too, has an interest in ending the bloodshed in Syria. The Russian public is as averse to embroilment in foreign wars as any public anywhere. The Kremlin may be keen to retain a foothold in the Middle East – if only to prevent its post-Soviet reach contracting further – but not, it has made clear, at any cost.

Moscow’s priority in Syria has been consistent: it is not to shore up the power of President Assad personally, but to prevent an Iraq or Libya scenario in the event of him being precipitately removed. The collapse of the Syrian state, which would threaten chaos closer to Russia and ease the movement of jihadis to and from its borderlands, is the Russian nightmare.

Russia’s hold over Assad is debatable, as is US influence even over its Nato ally Turkey

But if Russia’s objective in Syria has remained the same, Moscow’s tone, internationally, has been changing. There is a new preference for diplomacy and the exercise of “soft power”, as this latest move suggests.

Still, for all this, the auguries are not good. The last truce – announced seven months ago and designated more modestly as a pause – fizzled out. Russia’s hold over Assad is debatable, as is US influence even over its Nato ally Turkey, let alone the various armed groups operating with western support.

A recent meeting of Syrian opposition representatives in London – designed to show moderate politicians in waiting – produced a vision of vapidity and incoherence. Then, even as Kerry and Lavrov talked in Geneva, UK ministers launched broadsides against Russia’s role in Syria that hardly suggested western singleness of purpose.

Kerry’s peace-mongering is not without challenge, either. Elements in the Pentagon are apparently willing any deal with Russia to fail. Their hope is for a more hawkish President Clinton in the New Year.

It is clear that any outside initiative to reduce the fighting in Syria needs a more united will than currently exists. The Kerry-Lavrov plan offers a start. Let’s hope it is not a false one.