Exactly what Russia intends with its rapidly expanding deployment of troops, tanks and combat aircraft in the Assad family heartland on Syria's northern coast is difficult to discern, according to military experts and U.S. officials, who say they were not consulted on the Russian moves and were caught off guard by the intervention.
Already, however, the Russian move has thrown into disarray three years of U.S. policy planning on Syria, derailing calculations about how the conflict will play out that may never have come to fruition and now almost certainly won't.
Foremost among those was the expectation, frequently expressed by officials in the Obama administration, that both Iran and Russia would eventually tire of supporting the embattled Syrian regime and come round to the American view that Assad should step down as part of a negotiated transition of power. The conclusion of the nuclear talks with Iran in July further raised hopes that Washington and Tehran would also find common ground on Syria.
Instead, the arrival of hundreds of Russian marines, sophisticated fighter jets and armor at a newly expanded air base in the province of Latakia appears to signal a convergence of interests between Moscow and Tehran in support of Assad.
The intervention has given the regime a much needed boost at a time when government loyalists had been losing ground to the opposition, and has been broadly welcomed by Syria, Iran and its allies.
"The Americans thought that the negotiations with Iran could include a bargain on Syria, but this issue is over," Hasan Nasrallah, the leader of Lebanon's Hezbollah militia, whose fighters have been instrumental in securing Assad's hold on power, said in a television interview Friday. "The negotiations were only about the nuclear issue."
The sophisticated new weaponry being introduced by the Russians will give a qualitative edge to Assad's depleted and wearied government forces, thwarting, at least for now, predictions that his demise could be imminent, according to Chris Harmer of the Washington-based Institute for the Study of War.
"This extends Assad's lifetime indefinitely," he said. "As long as he's got major state based support from Iran and Russia he can survive."
The intervention also risks prolonging, intensifying and perhaps expanding the war, if, as is widely predicted, the Russian force uses its firepower not against the Islamic State but against the rebels seeking to topple Assad, some of them backed by the United States.
Russian officials have portrayed the deployments as part of a new effort to fight the Islamic State, amid growing doubts about the efficacy of the Obama administration's faltering strategy. U.S. plans to train and equip a Syrian force to battle the extremist group have turned into an embarrassing failure. A year-long campaign of airstrikes has had no evident impact on the Islamic State's control over its core territories.
Russian President Vladimir Putin has proposed the formation of an international coalition to battle the extremists that includes Iran and Syria and would presumably be led by Russia, accompanied by a peace process based out of Moscow in which Assad would play a leading role. The proposals, which Putin is expected to elaborate in a key address to the United Nations next week and in talks with President Obama on Monday, raise the specter of two rival coalitions against the Islamic State and two rival and contradictory peace processes.
Speaking after meetings with Russian officials in Moscow earlier this week, Hossein Amir-Abdollahian, Iran's deputy foreign minister for Arab and African affairs, left little doubt where Iran's inclinations lie.
"Iran and Russia are the serious and main partners in a peaceful settlement of the crisis in Syria," he said, adding that "Bashar al-Assad, the legitimate president of this country, should be part of the negotiations about Syria's political future."
How far Iranian and Russian interests coincide in Syria isn't clear.
Until evidence of the Russian deployments began to emerge in August, Iran had been the single most influential foreign power in Syria. Russia has consistently supplied weapons and equipment to the Syrian Army throughout the war, but it was Iran that stepped in with the cash and men needed to face down the rebellion. Iran's Revolutionary Guard Corps has sent advisers to fight alongside Syrian troops, and Iranian backed and funded Shiite militias, notably Hezbollah, have proved instrumental in sustaining Assad's hold over the capital, Damascus.
A series of recent visits by Iranian officials to Moscow, including one in July by Qasem Soleimani, the head of the Revolutionary Guard Corps' powerful Quds Force, preceded the Russian deployments, suggesting that they have taken place in consultation with Iran.
Yet the Russian intervention challenges Iranian influence in Syria almost as much as it does the relatively limited role played by the United States, said Marc Pierini of the Carnegie Endowment for International Peace.
"Russia is also making a statement vis-a-vis Iran," he said. "The Russian deployment in Syria is a way to counterbalance Iranian influence."
Iran has not lent its support to the Russian coalition, and at the United Nations on Friday, Iranian President Hassan Rouhani appeared to rule out that it would. "I do not see a coalition between Iran and Russia on fighting terrorism in Syria," Rouhani told a group of editors from news organizations.
Iran can be expected however to accommodate Russia's intervention for roughly the same reasons that the United States was hoping it would turn against Assad - because backing the Syrian regime has become a costly strain, according to Kamel Wazne, a Beirut-based political analyst who is close to Iran.
"The Russians are saying to Iran and Hezbollah, we are the decision-maker in this part of the world," he said. "Iran and Hezbollah will welcome this initiative because the war in Syria has been a drain."
"There have been a lot of casualties for Hezbollah in Syria and there is no end in sight," he added. "If you have a headache someone comes and says they will take the headache, you'll say, okay you can have it."
How much of the headache Russia will take away from the stretched loyalist forces has yet to be seen, military analysts say. Both Russia and Syria deny that Russian troops are participating in ground operations, though officials in both countries haven't ruled out that they might.
Yet the deployments are continuing, with aircraft and equipment arriving on a daily basis, according to U.S. officials and satellite imagery. Images obtained in recent days by the defense consultancy IHS Janes contain evidence that Russia is expanding its footprint at two new sites north of the airport in Latakia, including the construction of new buildings and tents of the kind used by Russian military units.
State Department spokesman John Kirby said last week that the intervention will be welcome if Russia is looking to play "a constructive role" against the Islamic State, but not if it is aimed at propping up Assad, "because it is the Assad regime that has been a magnet for extremists inside Syria."
Most analysts doubt the Russian engagement will become as deep or as intrusive as the intervention in Afghanistan in the 1980s, when Russian troops became deeply mired in a bloody war of attrition with U.S.-backed Afghan rebels that lured jihadi fighters from around the globe.
But almost any combat the Russians do engage in is are as likely to bring them into conflict with the rebels seeking to topple Assad as with the Islamic State, located hundreds of miles to the east of the Russians' coastal bases.
"The Russians will fight everyone who fights against Assad," said Wazne, the Beirut-based analyst. "They don't see a difference between extremists and the opposition."
Whether the intervention will work to end the war is a different question, he said. "If there is no political solution the conflict will drag on for a very long time. But the boundaries of the conflict will be clear, and Assad will be protected by the Russians."
