Turkish Lt. Gen. Erdal Ozturk, second from left, and others listen as President Obama speaks during a meeting with more than 20 foreign defense ministers Tuesday at Andrews Air Force Base in Maryland. (Evan Vucci/AP)

Two months after the start of its campaign against the Islamic State, the U.S.-led coalition conducting operations in Iraq and Syria has expanded significantly but remains beset by lingering strategic differences that threaten to undermine the fight.

The Obama administration has emphasized the breadth of the coalition it has assembled to combat the militant group, including the participation of five Arab countries that have played a supporting role in the campaign of airstrikes in Syria. But serious disagreements remain, particularly over the coalition’s plan for Syria and whether the fight against Islamic State militants there will strengthen or weaken Syrian ruler Bashar al-Assad in the long run.

Military chiefs from the United States and 21 other countries convened Tuesday for an unusual session at Joint Base Andrews in Maryland to discuss the campaign. The day-long event, hosted by Army Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, included an appearance by President Obama — part of an effort by his administration to dispel doubts about Washington’s long-term commitment to the region.

In his remarks to a room filled with military brass from around the world, Obama cited some preliminary “important successes” against the Islamic State but warned that “this is going to be a long-term campaign” with “periods of progress and setbacks.”

Sixty countries are now participating to some degree in the U.S.-led coalition against Islamic State forces in Iraq and Syria, according to the Obama administration. A number of administration officials have used the Bush-era term “coalition of the willing” while emphasizing that members may differ widely on what they are willing to contribute.

President Barrack Obama told military chiefs from 21 nations he is "deeply concerned about the situation in and around the Syrian town of Kobane.” (Reuters)

The question is whether such a diverse coalition, whose members have differing objectives, can be herded into agreement on a coherent strategy.

Turkey, which joined in the talks Tuesday but unlike most countries did not send its highest-ranking general, has so far balked at joining in the fight in Syria or allowing the U.S. military to use its nearby bases for airstrikes.

France and Turkey, in turn, have pushed to establish a no-fly and buffer zone to protect refugees in northern Syria, an idea opposed by the United States, Germany and others. No country in the coalition is eager to send ground troops to Iraq or Syria, despite a shortage of reliable proxy forces in either country to fight the Islamic State head-to-head.

Shadi Hamid, a Middle East scholar with the Brookings Institution in Washington, called Tuesday’s military summit “a positive step” in terms of coordinating tactical operations against the Islamic State. But he said it was unlikely to resolve deeper divisions, especially regarding desired outcomes for Syria’s long-running civil war.

“The coalition partners have very different conceptions about the regional order and don’t even agree on what the primary threat is,” he said. “You have all these different actors who want different things and in some cases also strongly dislike each other.”

Richard Fontaine, president of the Center for a New American Security, a think tank in Washington, said the Obama administration has successfully built the coalition based on the singular goal of defeating the Islamic State. But he said unity is likely to fray once other questions arise, such as whether to extend the fight to the Syrian government or other jihadist groups in the region.

Combating the Islamic State is “the unifying mission that all these coalition partners agree on,” he said. “Where it gets tricky is when you have to make other decisions.”

There have been mixed results on the battlefield since the U.S. military began airstrikes in Iraq on Aug. 8 and extended its air campaign to Syria on Sept. 22, with the Islamic State losing control of territory in some places while making gains in others.

In Iraq, U.S. warplanes and Kurdish troops on the ground have pushed back Islamic State fighters from the Sinjar mountain range, from the Mosul Dam — which controls much of the country’s water supplies — and away from the northern cities of Irbil, Kirkuk and Amerli.

But Islamic State forces have surged across Anbar province in western Iraq and over the weekend closed within several miles of the strategically vital Baghdad international airport, where hundreds of U.S. troops are stationed.

Army Col. Steve Warren, a Pentagon spokesman, said it was too soon to assess the effectiveness of the air campaign and the broader strategy, which U.S. officials have said will probably take years to unfold. “It’s simply too premature,” he said.

“It is a tough fight in Anbar, there’s no question about it,” Warren added. “There will be ebbs and flows across the battlefield for months.”

In Syria, the U.S. military and allied warplanes from Saudi Arabia, Jordan and the United Arab Emirates at first concentrated their airstrikes on Islamic State supply lines, small oil refineries and command centers.

In recent days, however, the coalition has been forced to shift its air operations to Kobane near the Turkish border, where Islamic State fighters have encircled the predominantly Kurdish city and threatened to massacre civilians. Since playing down the strategic importance of Kobane last week amid criticism that it was slow to intervene, the Pentagon and Arab countries have escalated their attacks there, including a blitz of 21 airstrikes on Monday and Tuesday.

The battle for Kobane illustrates one of the numerous fault lines running through the coalition. Syrian Kurds defending the town against the Islamic State are allied with the Kurdish Workers Party, or PKK, an enemy of Turkey. The Turks have refused to allow aid and fighters to cross the border to assist the Kobane defenders, believing it would strengthen the PKK.

While U.S. officials have gingerly noted that the survival of Kobane is not a strategic goal of the coalition, the administration has faced international warnings of a genocide if the Islamic State is allowed to overrun the town.

On Tuesday, French President François Hollande called it “a martyred town, a symbolic town” and said that “all countries concerned” should be providing weapons to Kobane defenders. “Turkey must absolutely open its border,” Hollande said.