Lt. Gen. William Mayville said at a Pentagon briefing on Tuesday that Americans should think of the campaign against the Islamic State as taking years. Here are key moments from that briefing in less than three minutes. (Sarah Parnass/The Washington Post)

Lt. Gen. William Mayville said at a Pentagon briefing on Tuesday that Americans should think of the campaign against the Islamic State as taking years. Here are key moments from that briefing in less than three minutes. (Sarah Parnass/The Washington Post)

U.S. military leaders said Tuesday their aerial bombardment of Syria was only the beginning of a prolonged campaign that will continue intermittently for months and will become more difficult as targeted militants seek refuge in populated areas.

The United States is now attacking two sets of enemies in the region: the Islamic State, a growing movement of jihadists seeking to create its own country in the Middle East, and the Khora­san group, a smaller network affiliated with al-Qaeda that officials say is plotting against Europe and the United States.

At the same time, as the U.S. military and its Arab partners prepare more airstrikes in Syria in the coming days, they will have to contend with another adversary: Syrian President Bashar al-Assad.

Although U.S. officials said they were not targeting Assad or his forces, many of the Sunni Muslim allies in President Obama’s coalition would like to see Assad’s government finally collapse after a devastating civil war. Whether the coalition’s intervention in Syria will eventually help or hurt Assad represents one of the greatest unknowns in a military campaign filled with uncertainty.

Army Lt. Gen. William C. Mayville Jr., director of operations for the Joint Staff at the Pentagon, said the objectives set for the U.S.-led war in Iraq and now Syria could take years to complete. The attacks in Syria marked the start of a new phase, coming six weeks after the U.S. military began a similar campaign of airstrikes against Islamic State fighters in neighboring Iraq.

The overarching goal, Mayville said at a news conference, is “to degrade and ultimately destroy” the Islamic State, first by directly attacking the group in both countries, severing its supply lines and disrupting its sources of arms and money.

To succeed over the long term, Mayville added, the United States and its allies will have to train and equip three proxy forces: the Iraqi military, the Kurdish pesh merga fighters and moderate Syrian rebels. All have proved hapless in head-to-head battles with the Islamic State and will require U.S.-military backing to reclaim territory.

“The most important thing is to create some space for the Iraqi security forces to reorganize and replace leadership that needs to be replaced,” Mayville said. “What we have been doing over these last couple of weeks and what last night’s campaign was about was simply buying them some space so that they can get on the offensive.”

In confronting the Khorasan group, a network that had received scant public mention before this month, the Obama administration may find itself acting largely on its own.

The first wave of strikes, which took place on what was late Monday in Washington, primarily consisted of dozens of Tomahawk cruise missiles launched by U.S. warships against eight Khorasan group targets in northwestern Syria, near Aleppo. Military officials said they attacked Khorasan training camps, a munitions production center, a communications building and other sites.

Mayville justified the blitz by citing U.S. intelligence reports showing the Khorasan group “was in the final stages of plans to execute major attacks against Western targets and potentially the U.S. homeland.”

He did not elaborate on those reports and declined to comment when asked whether there was evidence that any plots had been set into motion. He also said the military was still assessing the overall damage it inflicted on the Khorasan group and whether its ability to carry out attacks beyond the region had been diminished.

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The strikes against the Khora­san group were an exclusively American operation. Mayville did not explain why Arab allies did not participate in that phase, but other U.S. military officials noted that it would have been difficult for Arab warplanes to bypass air defenses and cross deep into Syrian airspace to reach Aleppo.

It is also unclear whether the five Arab partners consider Khorasan — which refers to an ancient Islamic region that covered parts of present-day Iran and Afghanistan — to be as grave a threat to their national interests as the Islamic State.

Warplanes from Saudi Arabia, Jordan, the United Arab Emirates and Bahrain participated in the second and third wave of airstrikes against Islamic State targets in north-central and eastern Syria, near the Iraqi border, according to Pentagon officials. Qatar also sent military aircraft in a supporting role, they said.

U.S. military officials said they conducted two more airstrikes Tuesday against Islamic State targets in Syria.

Mayville was reluctant to describe how substantial a role the Arab countries played, acknowledging only that the U.S. military dropped “a preponderance” of the ordnance.

All told, about 200 missiles, bombs and rockets were launched against 22 separate targets in Syria on Monday, U.S. military officials said.

In comparison, since Aug. 8, the U.S. military has conducted airstrikes against 194 Islamic State targets in Iraq. U.S. officials said that they expected the pace of airstrikes in both countries to ebb and flow in the coming days but that Monday’s attacks on Syria probably represented a high point.

The Pentagon deployed a mix of aircraft, including B-1 bombers, armed Reaper drones and several types of fighter jets Monday. Among them was the F-22 Raptor, an advanced stealth fighter that has been in the Air Force’s fleet for a decade but until Monday had never been used in combat.

The Tomahawk cruise missiles were launched by two warships, the USS Arleigh Burke in the Red Sea and the USS Philippine Sea in the Persian Gulf. Also participating in the airstrikes were Navy F/A-18 Hornet fighters flying from the USS George H.W. Bush, an aircraft carrier in the Persian Gulf.

In a sign of Arab nations’ sensitivity about their participation, the Pentagon refrained from saying where the other U.S. warplanes and drones that attacked Syria are based. The vast majority are stationed at installations in the Persian Gulf, including several large air bases in Qatar, the UAE and Kuwait. Although the existence of the bases is an open secret, those countries have asked Washington to keep the arrangements quiet.

The allied command center for the complex air operations was at al-Udeid Air Base in Qatar, U.S. officials acknowledged.

The U.S. military also has warplanes and drones stationed at a major joint base in Turkey, a NATO ally that shares a long border with Syria and Iraq. But Turkey has balked at allowing its territory to be used for airstrikes in either country.

The Pentagon released before-and-after reconnaissance photographs of a handful of selected targets in Syria to demonstrate how American forces relied on precision-guided munitions to avoid causing unnecessary damage.

Mayville said he was unaware of any credible reports of civilian casualties. He said the strikes had not intentionally targeted individual enemy commanders, but he acknowledged that such leaders had been “routinely” present at some of the targeted sites.

Prior to Monday, the Syrian government had repeatedly warned the United States and other countries not to violate its sovereignty by launching strikes. At the same time, there were clear signs that Assad welcomed the attacks on his foes.

Rear Adm. John Kirby, the Pentagon press secretary, said the United States did not coordinate the strikes with Assad’s government, although Washington did inform the Syrian government through its ambassador to the United Nations that the strikes would be carried out at some point.

Mayville said that Syrian military radar was “passive” in the face of the airstrikes and that Assad’s forces made no attempt to defend Syrian airspace.

In brief remarks outside the White House on Tuesday, Obama stressed that “this is not America’s fight alone” and vowed to press the battle against “these terrorists” in concert with U.S. allies.

“The overall effort will take time,” Obama said on the South Lawn. “There will be challenges ahead. But we are going to do what’s necessary to take the fight to this terrorist group.”

After his remarks, Obama boarded his Marine One helicopter for the first leg of a trip to New York to attend the opening of the U.N. General Assembly. He said he would meet there with the new prime minister of Iraq and with other friends and allies opposed to the Islamic State.

William Branigin contributed to this report.