WASHINGTON >> For the first time in the four-year Syrian civil war, President Barack Obama is beginning to execute a combined diplomatic and military approach to force President Bashar Assad to leave office and end the carnage.

As 50 Special Operations troops arrive in Syria to bolster the most effective opposition groups, the administration is gambling that Secretary of State John Kerry will have more leverage to push Russia, Iran and other players toward two objectives: a cease-fire to limit the cycle of killing and the establishment of a timeline for a transition of power.

But the task is enormous, given the number of nations and rebel groups operating at cross-purposes and the tiny size of the U.S. force so far. Even senior members of the administration express doubts in private about whether the effort is sufficient.

While Kerry has been optimistic that diplomacy can end the carnage, and has criticized the White House internally for moving too slowly, he has conceded doubts to aides that his strategy of fast-paced diplomacy can harness so many fiercely opposed forces toward a political solution.

Kerry will travel to Vienna this weekend, summoning many of Syria’s neighbors and European powers to turn a vague declaration of principles, settled on two weeks ago, into a plan for a political agreement. The talks, at least so far, have not included representatives of Assad’s government or the fractured rebel groups seeking to depose him, and Kerry is struggling to bring them to the negotiating room.

But after years in which the Obama administration has been accused of largely sitting on the sidelines while a quarter-million Syrians died and millions more were displaced, Kerry seems buoyed by being at the center of the only diplomatic effort underway.

"I can tell you that as long as he thinks there is a glimmer of possibility, he will not let go," Wendy R. Sherman, Kerry’s undersecretary for policy until six weeks ago, said recently.

The move to increase pressure to create negotiating leverage echoes the strategy that the administration employed to force Iran into negotiations that led to a nuclear deal last summer. That effort, which involved economic sanctions, a bolstered military presence in the Persian Gulf and covert action, produced perhaps the most complex arms negotiations in U.S. history.

But in the Iran case, most of the world’s major powers were in agreement. On Syria, there is no such unanimity.

Russia and Iran remain Assad’s two greatest protectors, though U.S. officials detect a division on the question of whether Assad must stay in power. The Russians appear to have little faith that Assad can hold on, while the Iranians have dismissed the idea of looking for an alternative to him, though that may simply be posturing.

On the other side of the table, America’s partners are also divided. Saudi Arabia had to be dragged to the negotiations by Obama and Kerry, after saying that a deal with Iran, its regional enemy, would be a fruitless exercise, and that Obama was being manipulated by the Iranians and the Russians in a bid for time.

Saudi Arabia and other Persian Gulf countries have largely ceased participating in the air war over Syria, focusing instead on fighting insurgents in Yemen. Privately, many of the foreign ministers who attended the last Vienna session two weeks ago questioned whether Syria could survive as an independent country. Many intelligence analysts providing reports to Obama and Kerry have expressed similar doubts.

"I think this is not a strategy — it’s more like a line of effort," said Andrew J. Tabler, a Syria expert at the Washington Institute for Near East Policy. "There are different lines of effort now, all part of the evolution of the great game of Syria."

For Kerry, the Syria effort has become an obsession, much like his failed bid to strike an Israeli-Palestinian accord in 2013 and his so-far successful effort on the Iran nuclear deal this year. On a trip through Central Asia last week, he used every spare moment to make phone calls from his plane or hotel to keep the diplomatic effort on track.

The Syria situation "may or may not be ripe" for solution, Kerry told students at the John F. Kennedy School of Government at Harvard before he left on the trip. "But it’s imperative. It’s a human catastrophe, a disaster that screams at all of us in public life to exercise responsibility in trying to find a solution."

At White House meetings, administration officials say, Kerry has argued that without applying both military and diplomatic pressure, the Russian intervention that began in late September would shore up Assad "and actually wind up destroying Syria."

But the question is whether the military element of the strategy — which reversed Obama’s previous determination to keep U.S. combat troops out of the country — will be enough to make a difference.

"What he is doing will not work," Fareed Zakaria, a journalist whom Obama frequently speaks with, wrote in a column in The Washington Post last week. "In a few months," he predicted, the United States "will face the challenge again — back down or double down. So far, President Obama has responded each time with increased intervention."

Reflecting that willingness to do more, Defense Secretary Ash Carter said on Sunday that additional troops could be sent to Syria if the United States could find more local forces to work with.

The White House is hoping that the Syrian, Arab and Kurdish coalition, aided by U.S. pilots and special operators on the ground, can seize and hold territory. The United States also plans to step up airstrikes. Obama deployed 12 A-10 warplanes and is sending additional F-15 fighter jets to Incirlik Air Base in Turkey to carry out strikes.

The theory is that over time the combination will increase the pressure on Assad to step down. The immediate focus is not on Assad’s strongholds, though; it is on territory seized by the Islamic State, including its capital, Raqqa.

But for the meeting this weekend in Vienna, and then for the Group of 20 summit meeting that Obama will attend early next week in Turkey, there is a far more immediate problem: identifying friends and enemies on the ground in Syria.

Several rebel groups aided by the United States have been bombed by the Russians in recent weeks. U.S. officials say they believe that Russia is trying to wipe out Assad’s opponents while claiming that it is going after the Islamic State, also known as ISIS.

Eventually, U.S. and Russian officials will have to share lists about the groups that they support and that could be credible interlocutors for a cease-fire and political solution. But the United States has declined to do so, for fear that the Russians or Assad would immediately target the groups for attacks.

Then would come the practical problem of engineering a cease-fire and a referendum to replace Assad. The draft of principles released two weeks ago said that any election must include Syrians outside the country, which would make it more difficult for Assad to rig the results.

The 50 Special Operations troops, U.S. officials said, will attach themselves to commanders of the tribal affiliates in northern Syria and coordinate with the Kurdish militia known as the YPG. That group has dealt the Islamic State its most significant setbacks across an enormous strip of northern Syria near the Turkish border.

"You’ve got the elements of a combined political and military strategy," said Derek Chollet, a former assistant secretary of defense in the Obama administration. "That, to me, is the key part — beating ISIS for our own interest, but changing the complexion of the Syrian opposition so that the moderates are the more powerful ones. That is key to getting Assad out."