A landmark agreement will see the US and Russia teaming up to tackle the twin threats of ISIS and Al-Qaida, it has been announced.
Both countries announced a ceasefire after a day-long meeting between secretary of state John Kerry and his Russian counterpart, Sergey Lavrov, concluded early Saturday morning in Geneva, Switzerland.
If the ceasefire, which begins Monday, holds for a week, the two countries will then begin an unlikely military partnership to target Islamic extremists in the region.
At a joint news conference, Kerry said the plan could reduce violence in Syria and may lead to a long-sought-after political transition.
It could also, he says, end more than five years of bloodshed that has claimed the lives of 500,000 people.
Kerry called the deal a potential 'turning point' in the conflict - although it will require support from both Syria's Russian-backed government, led by President Bashar Assad, and US-supported anti-Assad rebels.
The arrangement hinges on Moscow pressuring Assad's government to halt all offensive operations against Syria's armed opposition and civilian areas.
Meanwhile, Washington must persuade 'moderate' rebels to break ranks with the Nusra Front, Al-Qaida's Syria affiliate, and other extremist groups.
And both sides would have to pull back from demilitarized zones and allow civilian traffic and humanitarian deliveries.
That ceasefire and traffic must hold for a week before collaboration between the countries begins.
The US and Russia will then begin intelligence sharing and targeting coordination, while Assad's forces, no longer permitted to target Nusra, will focus their fire on ISIS.
Kerry's negotiating partner, Russian foreign minister Sergey Lavrov, said the deal could help expand the counter-terrorism fight and aid deliveries to Syrian civilians.
'This is just the beginning of our new relations,' he promised.
He also said Assad's government was prepared to comply with the arrangement.
And for his part, Kerry said it would be 'wise' for opposition forces to separate completely from Nusra - a statement Lavrov hailed.
'Going after Nusra is not a concession to anybody,' Kerry said. 'It is profoundly in the interests of the United States.'
Both sides have failed to live up to previous bargains - but the promise of a united military effort means there is more at stake this time around.
The deal comes after months of diplomacy that included four meetings between Kerry and Lavrov since August 26 and a lengthy face-to-face between Presidents Obama and Putin in China.
It also comes a year after Obama chided Putin for Russia's military intervention in Syria.
That intervention, US officials said, was more about keeping Assad in power by attacking rebel forces than dealing with ISIS in the region.
But the level of US-Russian interaction has upset several leading national security officials in Washington, including defense secretary Ash Carter and national intelligence director James Clapper.
Though the deal has been heralded as a breakthrough, it still needs to be implemented.
Given that Aleppo has spent the last 40 days is a vicious firefight between government and rebel fighters, enforcing the ceasefire could prove very difficult.
And as with previous blueprints for peace, Saturday's plan appears to lack enforcement mechanisms.
Russia could, in theory, threaten to act against rebel groups that break the deal.
But if Assad bombs his opponents, the US is unlikely to take any action against him, given Obama's longstanding opposition to entering the civil war.
Kerry only appeared at the news conference after several hours of internal US discussions.
At one point, Lavrov said he was considering 'calling it a day' on talks, expressing frustration with what he described as an hours-long wait for a U.S. response.
He then presented journalists with several boxes of pizza, saying, 'This is from the U.S. delegation,' and two bottles of vodka, adding, 'This is from the Russian delegation
The ceasefire begins at sundown Monday, Kerry said, coinciding with the Muslim holiday of Eid al-Adha.