WASHINGTON >> For five years, President Barack Obama has steadfastly rejected the argument that the United States could intervene in Syria, alter the equation on the battlefield and avoid being sucked into a quagmire. Now, it appears that President Vladimir V. Putin has done exactly that.

The Russian leader’s announcement on Monday that he would withdraw the bulk of his forces from Syria not only caught the White House by surprise; it seemed to belie Obama’s regular warnings that Russia would be severely damaged by its military adventurism. And it reinforced the sense that Putin has managed to maintain the initiative in Syria against an American president who wants to keep the war at arm’s length.

The White House on Tuesday cautiously welcomed Russia’s latest move, even as it continued to criticize its intervention, and administration officials struggled to understand Putin’s motives for acting now.

“The Russia military intervention propped up Assad and only made it more difficult for that political resolution to be reached,” the White House press secretary, Josh Earnest, said, referring to Bashar Assad, the Syrian president. “If they continue to follow through,” he added, “then that would be a positive outcome.”

Administration officials cautioned that they would need to see further evidence of Russian equipment leaving Syria before they concluded that the withdrawal was genuine. On Tuesday, the Pentagon said the U.S. military had seen fewer than 10 Russian planes depart for Russia and no major troop movements.

Still, U.S. officials said there were reasons to believe that Putin would follow through. He has tied his credibility to the fragile cease-fire now in place, as well as to the peace talks that resumed this week in Geneva. He is eager to ease tensions with the European Union over a migrant crisis that the Europeans blame partly on him. And he is impatient with Assad, whose forces have been unable to hold territory in Syria’s west, despite Russian air support.

Putin, these officials said, had reached a turning point in his campaign, where the costs, domestically and internationally, of staying engaged outweighed the advantages. Moreover, the Russians had largely achieved their primary goal: preserving the Assad administration and giving Moscow a seat at the table for any political settlement.

For Obama, Russia’s decision is a relief insofar as it eases the pressure on him to increase U.S. support for Syria’s moderate opposition, something he has long resisted. The administration wants Russia to play a role in the political negotiations because it wields influence over the Assad government, and analysts predicted the withdrawal would force Assad to make concessions he was not otherwise willing to make.

But the announcement is a reminder that since September, when Russia thrust itself into the conflict, Putin has consistently seemed a step ahead of the United States on Syria. His withdrawal could also prolong the U.S.-led campaign against the Islamic State, also known as ISIS, because while Moscow trained most of its firepower against anti-government forces, it also struck Islamic State targets.

The Pentagon spokesman, Peter Cook, said on Tuesday that there continued to be Russian airstrikes over the last 24 hours, and those airstrikes were carried out against the Islamic State.

“Suddenly, the Russians are no longer as available for the fight against ISIS,” said Andrew J. Tabler, an expert on Syria at the Washington Institute for Near East Policy. “That would seem to put a longer timetable on this war and a greater burden on the United States and the West.”

Earnest said the United States had received no “direct advance notice” of the decision from Russia — wording careful enough that it did not dispel questions among analysts about whether Russia had coordinated its announcement with the United States. A senior administration official insisted there was no “strategic grand bargain” between the United States and Russia on Syria.

Putin made the announcement just before he got on the phone with Obama for a call that had been scheduled at the request of the White House.

Obama, in an interview last week with The Atlantic magazine, repeated his assertion that the Russian intervention was a blunder — one that betrayed weakness rather than strength. To argue that Russia was in a stronger position in Syria since it had intervened, he said, was to “fundamentally misunderstand the nature of power in foreign affairs or in the world generally.”

“They are overextended. They are bleeding,” Obama told The Atlantic’s Jeffrey Goldberg. “And their economy has contracted for three years in a row, drastically.”

Experts on Russia, however, said there was little evidence that Putin was withdrawing because of economic or political pressure. The Russian press has portrayed the operation as a victory — a “Mission Accomplished” — with relatively few Russian casualties.

“This doesn’t look like a quagmire to me,” said Angela E. Stent, a professor of government and Russia expert at Georgetown University. “If they had been more ambitious in their goals — to destroy the Islamic State or to keep Syria whole — the outcome might be different,” she added.

Those are the goals of the Obama administration, which is why some analysts said it made little sense to compare Russia and the United States. Russia intervened in Syria on behalf of a sovereign government, while the Obama administration’s debate has been over how aggressively to get involved in efforts to uproot that government. And the United States could never have taken part, as Russia did, in a campaign that resulted in the kind of civilian casualties inflicted by the Syrian government’s forces.

“He clearly set out to buy time for Assad, and that worked,” said Derek Chollet, a former State Department and White House official in the Obama administration, referring to Putin. “But he made the situation in Syria demonstratively worse. When you go in without scruples, it’s fairly easy to succeed.”

The Russians, Chollet noted, have a long record of surprising the United States with tactical moves, going back to their invasion of Afghanistan. But those moves are not always successful in the long run. Other analysts, however, said that in one respect at least, Russia’s limited operation exposed a fallacy in the president’s argument: that any military involvement in Syria would inevitably lead to deeper engagement.

“Syria doesn’t have to be a slippery slope,” Tabler said. “Putin actually demonstrated you could intervene, bomb, put troops on the ground, and still get out. They effectively changed the situation on the ground, and kept the regime from collapsing.”

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