On Dec. 19, the United Nationsinfo-icon Security Council unanimously called for UN officials to observe the stalled evacuation of thousands of residents and fighters from the last rebel-held districts in the city of Aleppo, a process that began four days earlier. With President Bashar al-Assad's regime and its allies regaining full control over Syriainfo-icon's largest city, the nearly six-year-old Syrian civil warinfo-icon is entering a new phase.

Assad and his allies - including Russiainfo-icon, Iraninfo-icon and various Shiainfo-icon militias from Lebanoninfo-icon and Iraqinfo-icon - had imposed a long siege, including air strikes and intensive shelling, on the rebel-controlled parts of Aleppo. Assad signalled that he would take advantage of his opponents' weakness, and move against other rebel-held areas in northern Syria.

But Assad's forces, which are overextended and depleted after years of fighting, have had trouble keeping control of territory in other parts of Syria. On Dec. 11, Islamic Stateinfo-icon militants recaptured the historic city of Palmyra, nine months after Syrian regime troops drove the jihadists out of the UNESCO Worldinfo-icon Heritage site, where they had terrorized residents and destroyed ancient monuments.

The fall of Palmyra shows that Islamic State is far from beaten - and it's poised to take advantage of Syrian, Russian and Iranian military resources stretched thin throughout Syria. It also underscores that USinfo-icon President-elect Donald Trumpinfo-icon faces a complex task in trying to keep his campaign pledge to defeat the group, especially in Syria.

After the fall of eastern Aleppo, there are signs of an emerging division of labour in Syria between the incoming Trump administration and that of Russian President Vladimir Putin. Russia would continue its intensive air strikes and logistical aid to help Assad recapture territory from rebels, while the United Statesinfo-icon would take the lead in the fight against Islamic State. On Dec. 10,

Defense Secretary Ashton Carter announced that the Pentagon would send 200 additional special forces to Syria - for a total of 500 US troops on the ground - to help train and advise Syrian opposition groups who are fighting Islamic State, especially around the city of Raqqa.

The Syrian civil war has expanded into a regional proxy war involving Russia, Iran, Turkeyinfo-icon, Saudi Arabiainfo-icon, Qatarinfo-icon and the United States. Russia and Iran, which are the Syrian's regime two main backers, have mainly targeted rebel factions opposed to Assad, rather than trying to dislodge Islamic State from its bastions. Soon after the war started in 2011, Turkey, Saudi Arabia and the United States began supporting various rebel groups fighting Assad's regime. More recently, Washington has intensified air strikes and deployed special forces to mobilize Syrian opposition groups, mainly led by Kurdish militias, to oust the jihadists from Raqqa.

The Obama administration has coordinated militarily with Russia to a limited extent, but it criticized Moscowinfo-icon for entering the war to prop up Assad and failing to devote significant resources to fighting Islamic State. Trump, on the other hand, has made clear that he doesn't see removing Assad as a US priority, and he signalled a greater willingness to work with Russia. Assad is clearly pleased with the new administration, declaring in a recent TVinfo-icon interviewinfo-icon: "If Trump can genuinely fight against terrorism, he can be our natural ally."

But from the Syrian and Russian perspective, that alliance means relying on Washington to lead the fight against Islamic State, which neither Assad nor his backers view as urgent as recapturing rebel-held territory.

Already, Pentagon officials say they would be prepared to strike Islamic State in Palmyra, if Russian and Syrian forces fail to retake the city soon. US officials say they're concerned that the jihadists acquired powerful weaponry when they captured the city from Syrian troops, including armoured vehicles and air defense systems. "If the Russians and the regime don't strike it, we will," Lt. General Stephen Townsend, the top US military commander in charge of the coalition against Islamic State, said at Dec. 14 news conference.

Pentagon officials cautioned that Syrian and Russian forces were too focused on the offensive against rebels in Aleppo, and they allowed Islamic State to easily recapture Palmyra. "They failed to consolidate their gains and got distracted by other things that they were doing and took their eye off the ball there," Townsend said.

As soon as he's inaugurated on Jan. 20, Trump will face a crucial decision: Will he continue the Pentagon's support and training for the coalition of Syrian rebel groups which is leading a ground offensive to oust Islamic State from its self-declared capital in Raqqa? That campaign began in early November with a mobilization of about 30,000 Syrian rebels to encircle Raqqa, and close off the jihadists' resupply routes for weaponsinfo-icon and fighters from neighbouring Iraq. The US-backed rebels are now about 15 miles from the city, but the battle could take months, as it has in the Iraqi city of Mosulinfo-icon, where tens of thousands of troops are trying to dislodge several thousand militants.

The fledgling Trump administration wants to avoid becoming mired in Syria's complicated war, and has signalled that it wants Russia to continue taking the lead. But other powers might try to drag Washington deeper into the conflictinfo-icon, or use it to project strength, or to distract Trump from other goals, such as his insistence on dismantling the Iran nuclear deal.

Iranian leaders are claiming a large stake of the military victory in Aleppo, and boasting that Assad's regime would have been unable to retake the city without support from Iran and its allies, especially the Lebanese Shi'ite militia Hezbollahinfo-icon. Some Iranian officials are also using the Aleppo triumph to send a message to the Trump administration about Tehraninfo-icon's influence in the region.

"Aleppo was liberated thanks to a coalition between Iran, Syria, Russia and Lebanon's Hezbollah," Yahya Rahim Safavi, a senior military adviser to Iran's supreme leader, said in Tehran on Dec. 14. "Iran is on one side of this coalition, which is approaching victory, and this has shown our strength. The new American president should take heed of the powers of Iran."

In another sign of weakened US influence over events in Syria, Russia reached a deal with Turkey allowing Ankara to exert control along the Turkish-Syrian border through its own troops and allied rebels. In exchange, Turkey did not help the rebels in Aleppo resist the Russian-Syrian offensive. That could become a template for other deals between Russia and Turkey, and it would further isolate the Syrian rebels in their remaining northern strongholds. On Dec. 16, Putin announced that he is working with Turkish leaders to organize a new series of Syrian peace talks without Washington's involvement.

For his part, Assad will be emboldened by his military success in Aleppo - even one so heavily dependent on Russia, Iran and other foreign forces - to continue taking a hard line and refusing a political compromise with the rebels. And thanks to Russian maneuvering, Assad and his allies can outsource the difficult fight with Islamic State to Washington.