The Obama administration's plan to raise a 15,000-strong rebel army in Syria has run into steep political and military obstacles, raising doubts about a key element of the White House strategy for defeating Islamic State militants in the midst of a civil war.
Pentagon concerns have grown so sharp that Defense Secretary Chuck Hagel sent a two-page memo to the White House last week warning that the overall plan could collapse because U.S. intentions toward Syrian President Bashar Assad are unclear, according to a senior defense official who read the memo but was not authorized to speak publicly.
President Obama has called on Assad to step down, but he has not authorized using military force, including the proposed proxy army, to remove the Syrian leader.
At a news conference Thursday, Hagel declined to discuss his memo to national security advisor Susan Rice, but he acknowledged that Assad has inadvertently benefited from more than five weeks of U.S.-led airstrikes against the Islamic State, one of the most powerful antigovernment forces in Syria's bitter conflict.
Secretary of State John F. Kerry sought to paper over the problem Thursday, telling a forum in Washington that the proposed proxy army "can have an impact on Assad's decision-making so we can get back to a table where we could negotiate a political outcome, because we all know there is no military resolution of Syria."
Rebel leaders in Syria say they would reject joining a U.S.-backed force that is not aimed at defeating Assad, their main enemy.
Senior U.S. military officers also privately warn that the so-called Syrian moderates that U.S. planners hope to recruit — opposition fighters without ties to the Islamic radicals — have been degraded by other factions and forces, including Assad's army, during the war.
It will take years to train and field a new force capable of launching an offensive against the heavily armed and well-funded Islamic State fighters, who appear well-entrenched in northern Syria, the officers say.
"We're not going to be able to build that kind of credible force in enough time to make a difference," said a senior U.S. officer who is involved in military operations against the militants and who asked for anonymity because he was not authorized to speak publicly. "We've watched the moderate opposition dwindle and dwindle and now there's very little left."
The Pentagon plan calls for putting 5,000 rebel fighters into Syria in a year, and 15,000 over the next three years.
It is the least developed and most controversial part of the multi-pronged U.S. strategy, which also includes near-daily airstrikes in Iraq and Syria, deployment of U.S. military advisors and other support to assist Iraqi government and Kurdish forces, along with attempts to choke off the militants' financing from oil sales and foreign donors.
When officers involved in high-level Pentagon deliberations in the summer raised concerns about building a rebel army from scratch, they were overruled by senior commanders, who warned that airstrikes alone would not defeat the militants, one of the officers said.
But Pentagon unease has intensified in recent weeks as Jordan and Turkey, two allies that the Obama administration is counting on to help train the proposed proxy force, made it clear that they are lukewarm to the plan, two U.S. officials said.
Washington and its allies are chiefly split over whether the proposed force should focus on reclaiming Syrian territory now held by the Islamic State militants, which is the U.S. priority, or should also battle troops loyal to Assad, the allies' main concern.
Turkey said this month that it would train a portion of the Syrian force, joining Saudi Arabia in training on its territory. U.S. officials don't expect to assemble the first group of "moderate" rebels, drawing them from inside Syria or from crowded refugee camps in nearby countries, until early next year at the earliest.
But Turkish officials have signaled that the rebels it trains would concentrate on battling Assad's forces, not Islamic State, once they return to Syria.
Jordan has not joined the training effort, although it hosts a separate, smaller, CIA-run operation for Syrian insurgents.
U.S. officials say greater involvement by Turkey and Jordan would allow them to increase the number who can be trained, and provide easier conduits for support and resupply when they return to Syria.
The dispute reflects the complex calibrations now in play as the Islamic State militants shake long-established political and military fault lines in the Middle East.
Most dramatically, perhaps, U.S. forces are now in at least tacit alignment with traditional enemies such as Iran and Hezbollah, the Lebanon-based militant and political group, against a common threat.
Syrian rebel leaders and Arab allies complain that the U.S.-led airstrikes have helped Assad by weakening one of his most powerful foes and enabling his army to step up attacks on other rebel factions.
A spokesman for the Free Syrian Army, an umbrella organization claiming to represent largely autonomous rebels in Syria, said fighters were incensed by the U.S. insistence on focusing entirely on Islamic State.
"They have forgotten that tens of thousands of civilians are suffering because of the regime," said the spokesman, who did not want his name published because it could endanger his family. "Our main cause is the regime, and that will remain our main cause."
A rebel commander, a defector from the Syrian army who also asked for anonymity, agreed. The U.S. plan "doesn't work for us," he said.
"They are concerned with ISIS … but we are concerned with the regime more than ISIS," he said, using one of several acronyms for Islamic State.
U.S. Central Command, which is overseeing the effort to build a Syrian force, says questions about its direction will be resolved once the fledgling program is underway.
"We are early on in this and there's much to be figured out," said Maj. Curtis J. Kellogg, a spokesman for Central Command.
Frederic C. Hof, a former special advisor to President Obama for Syria, said the U.S. plan "is going to be a tough sell" in Syria.
"You can always get people by providing weapons, ammo and pay, but your appeal to a large number of Syrians will increase dramatically if it is a force whose goal is eventually to govern all of Syria," not just beat one faction, he added.
The caution reflects, in part, a U.S. desire to reassure Iran, one of Assad's closest backers, that it is not seeking to oust him by force. If the U.S. backtracked on that promise, Iran might step up military support for Assad.
Tehran also could respond by using local Shiite militias to attack U.S. personnel or facilities in Iraq. The Iranian-backed Shiite militias in Iraq have coordinated their attacks on Islamic State with the U.S.-backed government in Baghdad.
"If we really focus on Assad, the Iranian piece of this coalition [against Islamic State] will fracture, and we will have Shia militants trying to target us," said the senior U.S. military officer.
The U.S. experience with proxy military forces is laced with disappointment.
The Kennedy administration backed a failed invasion of the Bay of Pigs in Cuba in 1961 after training a counterrevolutionary brigade. In the 1980s, the Reagan administration bankrolled the Contras in Nicaragua, who were unsuccessful against the Sandinistas' socialist revolution.
"We've helped arm insurgencies before," Bruce Riedel, a former CIA analyst who now is a senior fellow at the Brookings Institution. "Nearly all of them have been complete failures or marginal to the final outcome. But there was one spectacular success."
The CIA, working with Pakistan and Saudi Arabia, covertly poured $4 billion into arming a rebel force in Afghanistan from 1979 to 1989, helping them drive out Soviet forces. Riedel, who wrote a book about the undertaking, said the CIA operation hastened the fall of the Soviet Union and the end of the Cold War.
The Syrian rebel forces, with their fractured leadership and rival sponsors, bear similarities to the competing Afghan mujahedin factions during that war, Riedel said. If the U.S. can secure tight-knit partnerships with neighboring countries on training the rebels, it could also see success against Islamic State.
"There's no reason we can't do it again," he said. "But it doesn't happen overnight."
Cloud and Hennigan reported from Washington and Abdulrahim from Los Angeles.