Six years into the world’s grisliest war and worst humanitarian crisis, a ceasefire went into effect on Friday in Syria. Russian President Vladimir Putin, its co-mastermind, admitted that it is “fragile.” Two previous ceasefires—in February and September—lasted only weeks.The odds are stacked against this one bringing an end to all the fighting, since it deals with only one of multiple wars inside Syria, which have so far killed an estimated four hundred thousand people. But the current initiative—brokered by Russia and Turkey with Iranian support—is different from past efforts. It signals big shifts in the basics of the Syrian tragedy.

For now, Russia has “won” Syria. Moscow escalated its involvement, in September, 2015, by providing the government of President Bashar al-Assad the air power he needed to pummel the rebels. The pretext was to bomb the Islamic State; it was a ruse. Some ninety per cent of Russian airstrikes have been against the rebels challenging the Syrian government. Assad could not have regained control of Aleppo—the country’s largest city and former commercial center—earlier this month without Russia. And the rebel rout in Aleppo changed the strategic reality enough to allow the Russians to step in and take the diplomatic lead on behalf of its most important ally in the Middle East.

The initiative is a boon personally for Putin, who has been pressing on many fronts to restore Russia to a dominant place in global gamesmanship. He has seized the lead in resolving the biggest flashpoint in the twenty-first century. If the ceasefire holds, Russia and Turkey have said they will co-sponsor peace talks next month between the Syrian government and seven major armed opposition groups that are party to the ceasefire in Astana, the capital of Kazakhstan. The Russian foreign ministry posted a list of the seven groups—who have a total force of more than fifty thousand fighters—on its Web site, though the rebels themselves have said little.

The United States has effectively lost its goal—announced by President Obama after a popular uprising began in 2011—of forcing Assad to step down. Washington was deliberately excluded from the diplomacy and put in the awkward position of welcoming a ceasefire it had long sought but failed to achieve, largely because of Russia.

“News of a ceasefire in the Syrian civil war is a positive development. We hope it will be implemented fully and respected by all parties,” the State Department’s Deputy Spokesman Mark Toner said, on Thursday. “Any effort that stops the violence, saves lives, and creates the conditions for renewed and productive political negotiations would be welcome.” Another U.S. official told me that the United States does not have a lot of optimism that the ceasefire will hold, even though some of the steps proposed are auspicious.

Assad is now going to outlast President Obama—and may continue to rule Syria for some time if a compromise can be crafted with the weakened rebel forces.

The deal marks another turn: Turkey has effectively flip-flopped on Syria. President Recep Tayyip Erdoğan had been one of the first leaders to turn his back on Assad. He allowed Turkey to be the most important source of support for Syria’s rebels and the main haven for its fleeing refugees. The terms of the deal basically mean that Turkey is prepared to turn its back on most of the rebels it has backed for six years.

“The importance of this is not whether it’s a ceasefire that holds. It’s that Turkey has signed on with Russia, and ultimately Assad, to close the door on the rebellion,” Joshua Landis, a Syria expert at the University of Oklahoma, told me on Friday. “Turkey is trying to get back to normal. It wants a way out of its downward spiral. It’s a brutal world out there and Turkey is saying, ‘I can’t do this anymore. I gave you six years. It’s too much of a load to carry.’ ” Turkey has absorbed almost three million Syrian refugees.

“The arrangement aims to expand the ceasefire in Aleppo to other parts of Syria, secure uninterrupted humanitarian assistance and revitalize the political process,” Ibrahim Kalin, a spokesman for Erdoğan, said after the announcement on Thursday.

For Iran, the agreement protects its regional interests—including the so-called Shiite crescent that stretches from Tehran, through Baghdad and Damascus, to Beirut. Iran’s Quds Force, the most élite unit of the Revolutionary Guards, has played a key role in the fighting. Hundreds of Iranians, and at least a dozen generals, have been killed in Syria. Tehran’s most important ally is Hezbollah—and Syria is the geographic conduit and political protector of the Lebanese militia, which has as many as eight thousand fighters in Syria.

In a tweet on Friday, Iranian foreign minister, Mohammad Javad Zarif, said the ceasefire was a "major achievement. Let's build on it by tackling the roots of extremist terror."

The new relationship between Russia and Turkey—who were long at odds over Syria’s future—now squeezes various rebel groups, and cuts off their supplies and support. “Access, guns, aid—that back door to Syria through Turkey is now closed,” Landis told me. “The rebels are going to have to take care of themselves. They will have to go find some way to leave the country, sue for amnesty, go into peace talks, or fight to the bitter end.” The biggest rebel pocket left is in Idlib Province, in the northwest, on the border with Turkey. But they have been able to fight largely because of access to the outside world through Turkey.

The Syrian political opposition, which has received strong support from the United States and other Western powers, also appears to be marginalized in the new initiative. The talks include armed factions, not opposition politicians, many of whom have been exiled.

Even if the ceasefire holds in much of the country, the war against the Islamic State is not over. ISIS still holds significant territory in the north and east, despite recent losses to a local Kurdish militia. The ceasefire also does not include Al Qaeda’s most active branch, Jabhat Fateh al Sham, the rebranded version of the Nusra Front—which is based in Idlib Province. The Syrian regime, backed by Russian airpower and Iranian and Lebanese forces, is likely to continue the military campaign against the Al Qaeda franchise even as it is talking to other armed opposition forces in Kazakhstan.

There are also many smaller militias—the estimates of the disparate Syrian forces exceed one thousand—that are not signatories. They still control as much as forty per cent of Syrian territory, Landis said. “Assad is not going to be content until he owns it all,” he added. “A statement by the Russians, Turks, and Iranians agreed that they all want to respect Syrian sovereignty—or an undivided Syria.” So the war will flare on in a significant part of the country even if the guns fall silent in most of it.