President Vladimir Putin's announcement that Russia will begin withdrawing the "main part" of its military from Syria is a surprise potential end to a six-month intervention that bolstered Syrian President Bashar al-Assad and dealt a grave blow to Syrian rebels.
The decision came as United Nations-brokered peace talks between the Assad Government and rebel representatives got under way in Geneva.
The planned start of the withdrawal coincides with the five-year anniversary of the beginning of street protests in Syria, an initially peaceful movement that was brutally crushed by Assad forces.
Through it all, Russia has backed Assad. But the decision may intensify pressure on the Syrian Government to strike a deal with rebel groups in Geneva. Talks resumed there after breaking down a month ago because the rebels were suffering such heavy losses in their surrounded stronghold of Aleppo.
A shaky ceasefire has quelled fighting in Syria since late last month, but Assad's forces have continued an assault on their rivals.
"I hope that this will considerably increase the level of trust between all parties of the Syrian settlement and will contribute to a peaceful resolution of the Syrian issue," Putin said in a meeting with his top deputies that was broadcast on Russian state television. In a separate phone call with Assad, Putin said the intervention had "radically changed the situation" on the ground, the Kremlin said.
Putin said that Russia would keep open its air force and naval bases in Syria but that the task of the Russian intervention had been achieved and diplomacy should now take over.
The announcement took the Obama Administration by surprise and Barack Obama later "discussed" it with Putin in a phone call that had been scheduled to talk about the ceasefire, the White House said.
Putin made the decision unilaterally, without any such request from Assad, Kremlin spokesman Dmitry Peskov said. It was a pointed message suggesting that Russia's support for Assad is not unlimited, now that he is unlikely to be deposed by force.
It was not immediately clear whether the announcement meant a full end to all Russian airstrikes in Syria. The Kremlin spokesman said Russia did not believe that issues with "terrorists" - the term Russia generally uses for all of Assad's opposition - had been solved and that Russia intends to maintain a presence on the ground.
Previous Russian announcements about peaceful intentions in Syria have been met with scepticism by Western nations.
After Assad appeared weakened and on the verge of defeat mid-last year, the Russian intervention inverted the course of the conflict, paving the way with airstrikes for Assad's ground forces.
By February, shortly before the ceasefire went into effect, dozens of Russian bombers and jet fighters were often flying more than 60 sorties a day, according to Russia's Defence Ministry, enabling major territorial gains by regime forces. Although Russian leaders said they were targeting Isis (Islamic State) and other "terrorists", US officials and rebels said the bulk of the airstrikes were being conducted against other rebel forces battling Assad, some of which were supported by the US.
The mission was Russia's first overseas combat deployment since the 1991 breakup of the Soviet Union, a major test for a military that in 2008 seemed badly stretched by a brief war in neighbouring Georgia. Russia has sought to use the increased clout to play a bigger role at the negotiating table and to break through the international isolation that had settled on it after its 2014 annexation of Ukraine's Crimean peninsula.
Five years ago, few Syrians would have imagined that their uprising against their leader - a peaceful Arab Spring revolt - would turn into a violent proxy war for regional actors.
On March 15, 2011, Syrians took to the streets in Damascus for unarmed rallies that would spread across the country and would eventually be met with utter brutality by Assad's security apparatus. But Isis and al-Qaeda-affiliated Jabhat al-Nusra hijacked their revolt and provided the pretext for the Russian intervention.
For the first three months of the intervention, analysts and officials reported modest gains, as doubts about the battered Syrian army and other militias loyal to Assad persisted. But in January, a Syrian offensive began scoring major victories, cutting off supply lines from Turkey and threatening Aleppo.
After helping broker the late February ceasefire, Russia pledged that it would push Assad forces to adhere to the deal.
The US Administration had become increasingly frustrated in recent days over what it saw as Russia's inability or unwillingness to press Syrian Government forces to adhere to the ceasefire. The White House said President Obama, in his call to Putin, welcomed the overall reduction in violence but stressed that the continuing Syrian offensive risked undermining the truce and the political negotiations.
Late last week, the Administration decided to publicly accuse Moscow of failing to rein in Assad, with Secretary of State John Kerry saying Putin "should be somewhat concerned" by Syria's actions.
Russian analysts said the announcement may be intended to press Assad at the talks after saving him on the ground. "I think that Russia is really not interested to fully take the responsibility for this behaviour", said Fyodor Lukyanov, editor of Russia in Global Affairs.
Randa Sim, of Johns Hopkins, tweeted: "Surprising decision by #Putin. Its timing is interesting. It comes as a result of growing exasperation in Moscow with Assad."
Tim Ash, head of emerging-market strategy at Nomura in London, said: "Putin is just sending a message herein that he is 'in control' in Syria, and he can force Assad to the negotiating table, if or when he so wishes."
Putin is also trying "to reinforce on the West that Russia can brings lots of 'solutions' in Syria, and all this still seems to be a Russian negotiating ploy with the West".
Syrian opposition leaders offered cautious praise of the pullout decision. "For us, it's important to see actions instead of hearing words," said Salem al-Muslet, a spokesman for the main opposition group, the High Negotiations Committee.
It was unclear what effect the pullout would have on the negotiations.
The UN envoy charged with the talks, Staffan de Mistura, said ahead of Putin's move that "the only Plan B available is the return to war, and to an even worse war than we had so far".
- Washington Post, Bloomberg