U.S. officials are still debating the meaning and repercussions of Russia's move, though analysts at the Pentagon say it wasn't unexpected. The withdrawal might improve the prospects for peace talks in Geneva, some U.S. officials and experts say. More immediately, it's likely to add fuel to the debate -- particularly among presidential hopefuls -- about the U.S. role in Syria.
"For four years, the Administration stood by as the Assad regime slaughtered nearly half a million people in Syria," said Republican John McCain, chairman of the Senate Armed Services Committee, in a statement. "The Administration rationalized its indifference and inaction by saying that Russia would find itself in a quagmire."
Russian President Vladimir Putin announced the withdrawal as the United Nations peace talks on Syria began in Geneva on Monday. It came almost exactly five years after the start of anti-government protests that have metastasized into an international conflict, leaving at least 250,000 dead and sending nearly half the country fleeing from their homes.
The Russian withdrawal and the start of U.N. peace talks mark the beginning of "a very important phase in this process," Secretary of State John Kerry said Tuesday in remarks at the State Department.
Kerry said he would travel to Moscow next week to meet with Putin and Foreign Minister Sergei Lavrov "to discuss how we can effectively move the political process forward and try to take advantage of this moment."
"This is a moment to seize, not waste," Kerry said. "We have at this moment the ability to finally take a step towards ending this war and the bloodshed."
In a call with President Barack Obama Monday night, Putin said he was pulling his troops out because they'd fulfilled their "primary objectives" in the "fight against international terrorism," the Kremlin said.
That claim raised eyebrows in U.S. political and security circles, particularly as those officials say Russia targeted moderate opposition forces backed by the U.S. more than they did ISIS, whose threat has not been tamed despite Moscow previously stating the organization's demise as a key goal.
While there was strong agreement among Western diplomats and Washington foreign policy hands that Russia is getting out at this time because it has secured both its military interests in Syria and the government of close ally President Bashar al-Assad against opposition forces, U.S. and European officials pointed to other factors less agreeable to Moscow.
Pentagon officials have long said Putin could not keep up the pace of military operations given their high tempo and the fact that forces weren't being rotated. And European diplomats say that the Russians weren't entirely satisfied with the situation on the ground, in part because of the poor quality of Syrian regime troops they had to work with.
"We think they weren't making as much progress as they'd been hoping," said one diplomat who spoke anonymously to discuss sensitive matters. He added that the operations and particularly the air campaigns were costly for a Russian economy struggling with sanctions and lower-than-expected revenue due to falling oil prices.
Putin's decision to get out now "may be calculated to downsize Russia's role before the costs of the intervention become too steep or extrication more difficult," said Rep. Adam Schiff, a California Democrat and ranking member of the House Permanent Select Committee on Intelligence.
At the same time, Putin has reached some core military goals, securing control of a port traditionally used by Russia as well as an airbase.
The Hmeimim Air Base "was established after Russia's military campaign began last September, so it will certainly require Moscow to keep more forces in Syria than it had before the intervention, even if it does make good on its pledge to withdraw some units," noted Anna Borshchevskaya, a fellow at the Washington Institute for Near East Policy.
Both military assets give Putin the ability to resupply Assad and the Iranians without being there in as strong numbers as before, Pentagon officials say, and he can still strike from the Caspian Sea or southern Russia, or order troops back in.
Yet Pentagon spokesman Peter Cook noted Tuesday it wasn't clear how fully Russia is withdrawing.
"We've seen in the last 24 hours Russians continue to engage in some airstrikes in Syria, so I think obviously this is something we'll wait to see exactly what -- what transpires with regard to the Russians," Cook said.
But leaving now serves several purposes at home and abroad. Putin can eliminate domestic criticism of the Syria venture in advance of upcoming legislative elections, said Matthew Rojansky, director of the Kennan Institute at the Wilson Center, a Washington think tank.
"This way no one can campaign on a 'bring our boys home' nationalist platform, and no one has to defend against that charge," Rojansky pointed out.
Putin has also gained leverage over the regime, major influence over the Geneva peace talks and put Russians front and center to influence any new Syrian government if a political resolution is found, Pentagon officials say.
For now, Putin's departure may reflect "growing displeasure with Assad," Schiff said, and an attempt to "incentivize the Syrian government to negotiate an end to the conflict" instead of using the diplomatic process to stall and make more military advances.
"The consensus of Syria experts is that the civil war cannot end so long as Bashar al-Assad remains in power," Schiff said. "Now, with his protector leaving, Assad may have to be more amenable to a phasing out of his rule."
White House spokesman Josh Earnest said Tuesday that earliest indications are that the Russians are following through on the withdrawal. He added that while the administration had raised concerns that aggressive intervention could potentially remove any incentive for Assad to engage constructively in talks, it's still too early to tell what impact Russia's move will have on the broader situation.
But those on the other side of the aisle, especially the ones trying to succeed Obama in the Oval Office, see fewer positive outcomes from the recent moves and from the administration's overall policy toward Russia's presence in Syria.
Several GOP presidential hopefuls have repeatedly bashed the President for not doing more to counter Russia's presence in the war-torn country. They are unlikely to see its withdrawal now as a sign that Obama calculated correctly by not taking stronger steps against Moscow but instead predicted that it would face a "quagmire" from its military involvement.
Republican presidential candidates, echoing McCain, have seized on Syria to accuse the White House of weakness. "If you look right now, part of the problem is that Putin, I think, has taken the measure of the man in Barack Obama and doesn't respect him," Texas Sen. Ted Cruz told CNN on November 5, a few months after the Russian leader sent his troops into Syria.
Florida Sen. Marco Rubio, who is also vying for the Republican nomination, said at a Sept. 16 CNN Republican presidential debate that Putin "is exploiting a vacuum that this administration has left in the Middle East" and is trying to woo away traditional American allies in the region such as Egypt and Saudi Arabia.
"What he is doing is, he is trying to replace us as the single most important power broker in the Middle East," Rubio said, "and this president is allowing it."
Democratic candidate Hillary Clinton, who sparred often with Putin while secretary of state, has also said she'd take a tougher course with Russia than the Obama administration.
"I remain convinced that we need a concerted effort to really up the costs on Russia and in particular on Putin," Clinton said at the Brookings Institution in Washington on Sept. 9, discussing the Russian leader's actions in Ukraine. "I am in the category of people who wanted us to do more in response to the annexation of Crimea and the continuing destabilization of Ukraine."