The U.S.-led air assault across a wide swath of northern Syria — broadening the campaign from neighboring Iraq — was an audacious move, signaling a major escalation in the battle against an Al Qaeda breakaway faction formerly known as Islamic State of Iraq and Syria, or ISIS.
The strikes hit multiple targets, the Pentagon said, including "fighters, training compounds, headquarters ... storage facilities, a finance center, supply trucks and armed vehicles."
But six weeks of bombardment in Iraq, including almost 200 airstrikes, have resulted in relatively few territorial gains against Islamic State.
The momentum of the Sunni Muslim militant group has been blunted in Iraq, but the extremists still hold sway in much of the country's Sunni heartland. There is widespread agreement that some kind of ground offensive will be needed to oust them from their Iraqi strongholds, such as Mosul and Tikrit.
In Iraq, Washington at least has the support of allies on the ground, including the army, affiliated Shiite Muslim militias and the spirited if ill-equipped Kurdish peshmerga force, not to mention a functional government to work with.
In Syria, however, the U.S. has no reliable on-the-ground partner, only disparate "moderate" rebels who have steadily lost territory to Islamist militants.
After more than three years of war, much of Syria is lawless, a nation where hundreds of armed groups, proclaiming various degrees of Islamist militancy, hold sway. Though Iraq may pose a complex problem, it hasn't plummeted to the level of Syria's utter despair.
The fact that the Obama administration is preparing to overhaul its approach and vet, train and equip the so-called moderate Syrian rebels attests to its lack of confidence in the only opposition fighters it has backed so far: the loosely organized Free Syrian Army, more a shifting franchise operation than a unified fighting force with a coherent central command.
Analysts doubt that the White House's initial pledge of about $500 million will be sufficient to train and equip a new army to defeat the Islamists, who are flush with captured weapons and cash from oil smuggling and other illicit enterprises. Moreover, the Syrian rebels say their principal aim is to overthrow not the Islamists but the Syrian government, whose formidable military is backed by Iran and Russia.
In effect, this newly revamped, U.S.-backed rebel force, whenever it is ready to deploy — the buildup could take years — will be fighting a war against two powerful adversaries, Islamic State and the Syrian military. That equation hasn't generated much optimism.
"We spent hundreds of billions of dollars and years of effort trying to build up forces in Afghanistan and Iraq, and look at what we got for it," said Joshua Landis, a Middle East expert at the University of Oklahoma. "We don't have a partner in Syria. That's the reality of the situation."
It's not yet clear whether Washington's purported allies in Syria are completely on board with the U.S. offensive against Islamic State. One of the administration's favored moderate rebel factions, Harakat Hazm, part of the Free Syrian Army alliance and a recipient of U.S. missiles and training, issued a statement Tuesday denouncing the "external intervention" — that is, the U.S.-led bombing campaign in Syria — as "an attack on the revolution."
The group said its main goal was toppling Assad. It is demanding "unconditional arming" of the Free Syrian Army, yet its members also acknowledge fighting alongside Al Nusra Front, the official Al Qaeda force in Syria.
Still, the country's motley bands of fighters labeled as moderates may well be the White House's best hope for now. It has few other options.
Some analysts have urged the administration to forge a limited, conditional alliance against Islamic State with the government of Syrian President Bashar Assad, a kind of lesser-of-two-evils approach. Syrian warplanes have been pummeling Islamic State positions for weeks, and Assad has long presented himself as a bulwark against regional "terrorism."
But Obama, who called for Assad to step down in 2011, declared in a nationally televised speech Sept. 10 that "we cannot rely on an Assad regime that terrorizes its people, a regime that will never regain the legitimacy it has lost."
U.S. allies such as Saudi Arabia, Qatar and Turkey, all predominantly Sunni nations, have invested too much in removing Assad — a member of an offshoot sect of Shiite Islam — to turn away from that goal.
Washington already faces criticism from Sunni allies that it is backing a Shiite-dominated, pro-Iran government in Baghdad. Aid from Iran and its Lebanese ally, the militant group Hezbollah, has been crucial in keeping Assad in power in the face of Syria's largely Sunni uprising.
Other potential U.S. partners in Syria are few.
Candidates include the ethnic Kurds, who are among the most effective anti-Islamist forces in the country. However, the Syrian Kurdish militia known as the Popular Protection Force is viewed as suspect because of its close ties to the Kurdistan Workers' Party, a Turkey-based guerrilla group labeled a terrorist organization by Turkey and the U.S. At any rate, Kurds are a minority in Syria and their movement is largely limited to the north.
Yezid Sayigh, a senior associate at the Carnegie Middle East Center in Beirut, said that in both Syria and Iraq, Washington faces a dilemma in "finding partners who are not only able and willing to fight on the ground, but who will not generate political problems that could lead to even greater polarization."
That fundamental challenge is an imposing one in Iraq. In Syria, it seems beyond daunting.
Times staff writer Raja Abdulrahim in Irbil, Iraq, and special correspondent Nabih Bulos in Beirut contributed to this report.