After five years of bloodshed and hundreds of thousands of deaths, Syria is less than a day away from an unlikely ceasefire that could make or break international efforts to end the war.
In Washington, it is hard to find anyone who believes the guns will truly fall silent on Saturday, and US officials portray the truce as a test of Russia's true support for the peace process.
But, despite Secretary of State John Kerry's talk of a mysterious "Plan B" if the peace is shattered or talks on a political transition fail, the US has little leverage over a confident Russia.
Under a deal laboriously agreed by Presidents Barack Obama and Vladimir Putin, the unlikely co-sponsors of international peace efforts, a ceasefire is to come into effect early Saturday.
But, with confusion on the ground between the "moderate rebels" backed by the US, Turkey and Saudi Arabia, and the "terrorists" that all parties agree are still fair game, will the truce hold?
"The cessation of hostilities that will take effect at midnight tomorrow is a potential step in bringing about an end to the chaos," Obama said Thursday.
"Now, even under the best of circumstances we do not expect the violence to end immediately. In fact, I think we are certain that there will continue to be fighting," he admitted.
Obama warned that extremists like the Islamic State group will certainly fight on, but US officials also expect Russian and Syrian forces to breach the truce.
A senior US official told AFP "there's pessimism, not expectation, pessimism," and cited what he said was Russia's history of making then breaking ceasefire commitments in conflicts in Georgia and Ukraine.
Shadi Hamid, Middle East expert and a senior fellow at the Brookings Institution, was more blunt: "Even calling it a ceasefire is giving the process too much credit."
Kerry, who is in near daily contact with his Russian counterpart Sergei Lavrov, is marginally more upbeat, but still said the onus was on Moscow to stand by its agreement to support talks on a transition.
"I'm not here to vouch that it's absolutely going to work, but this is the one way that we can end this war," he told US lawmakers this week.
Kerry argues that Russia has an interest in promoting talks to avoid seeing its forces dragged into a drawn out counter-insurgency effort against the rebels, who could receive greater outside support.
This week, he has repeatedly floated the idea of a so-called Plan B to be put in effect if the US-sponsored negotiations between the regime and the rebels are still stalled in two months' time.
- Shoring up Assad -
Kerry has not detailed the new strategy, which would have to be approved by Obama, but privately officials suggest Plan B could involve greater US military intervention in Syria.
This could take the form of increased support and more advanced weaponry for rebels opposed to Syrian President Bashar al-Assad enduring the Russian aerial onslaught.
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Kerry would not confirm this publicly, but he implied that US allies Saudi Arabia, Qatar and Turkey could make life difficult for Russian forces by boosting their backing for the rebels.
"While Russia has succeeded in shoring up Assad, that doesn't end the process for Russia because Russia is there and on the ground, and holding territory is hard," he said.
"Turkey, Qatar, Saudi and others, the opposition, have made it very clear war will not end if Assad stays."
Some experts scoff at this analysis, however, arguing that Obama has no appetite for deeper US military intervention and Russia faces no credible threat if it continues to batter anti-Assad rebels.
"What real incentives do the Russians have to act in good faith?" Hamid demanded in an AFP interview.
"There is no credible threat of military force and without a credible threat of military force, the Russians are going to act like they have been acting for the past several years."
Hamid described the US administration's argument that Putin will realize the error of his ways if faced by a tougher insurgency as having "a level of absurdity which is awe-inspiring and mind boggling."
He argued that it was wrong for the White House to assume rivals like Russia would have the same perception of their national interests and that in any case Moscow's intervention was going well.
"We can now judge that it's been a major success and the Russians have been vindicated," he said.
- Great power politics -
In Moscow, independent analyst and commentator Alexander Golts said Russia might respect the ceasefire -- and compel Assad to follow suit -- as a nod to its return to great power politics.
"The ceasefire decision was agreed by the presidents of Russia and America -- which, as far as the Kremlin is concerned, is the normal order of things," he said.
But he too cited the example of Ukraine and warned that Putin and Assad might decide to press their advantage in the siege of the city of Aleppo even while expressing support for the peace process.
"If, under whatever pretext, Russia continues bombing and Assad continues his offensive, the ceasefire will be called into question," he said.
Before he was appointed to Obama's administration, Kerry favored imposing a no-fly zone to protect Syrians from the regime, and his predecessor at the State Department, Hillary Clinton, has spoken in favor of such a tactic on the campaign trail.
But the current resident in the White House is known to be reluctant to risk a new confrontation, limiting Washington's ability to credibly put pressure on Putin or even Assad.
"There's nothing to suggest that Obama himself is willing to consider any Plan B," Hamid said.