Suspected massacres by the Islamic State. (The Washington Post/The Institute for the Study of War, the Long War Journal, news reports)

The cost of turning against the Islamic State was made brutally apparent in the streets of a dusty backwater town in eastern Syria in early August. Over a three-day period, vengeful fighters shelled, beheaded, crucified and shot hundreds of members of the Shaitat tribe after they dared to rise up against the extremists.

By the time the killing stopped, 700 people were dead, activists and survivors say, making this the bloodiest single atrocity committed by the Islamic State in Syria since it declared its existence 18 months ago.

The little-publicized story of this failed tribal revolt in Abu Hamam, in Syria’s eastern Deir al-Zour province, illuminates the challenges that will confront efforts to persuade those living under Islamic State rule — in Iraq as well as Syria — to join the fight against the jihadist group, something U.S. officials say is essential if the campaign against the militants is to succeed.

The Abu Hamam area has now been abandoned, and many of the bodies remain uncollected, offering a chilling reminder to residents elsewhere of the fate that awaits those who dare rebel.

Just as powerful a message for those living under the militants’ iron fist was the almost complete international silence on the bloodbath.

News of the massacre coincided with President Obama’s decision to order airstrikes to turn back an Islamic State advance unfolding farther east in Iraq, toward the Kurdish regional capital of Irbil, as well as humanitarian airdrops to help desperate Iraqi Yazidis trapped on a mountain by the onslaught.

Many Syrians in the opposition are starting to complain about unequal treatment.

U.S. warplanes have carried out more airstrikes on Islamic State forces besieging the Kurdish town of Kobane on Syria’s border with Turkey than on any other single location in Iraq or Syria. And Washington announced Sunday that U.S. planes had airdropped weapons and medical supplies to the beleaguered Kurdish fighters there.

Yet even now, Washington has directed little effort toward helping Sunni Arabs who want to fight the militants but lack the resources to do so, said Abu Salem, who was among the Shaitat tribesman and rebel commanders who gathered recently in an apartment in the Turkish border town of Reyhanli to recount the killings of their clansmen.

“We saw what the Americans did to help the Yazidis and the Kurds. But they have done nothing to help the Sunnis against the Islamic State,” he said.

Abu Salem and the other men said they did not so much begrudge the efforts to help Kurds as wonder why no one had helped them when their community was under attack. The carnage inflicted on the Shaitat tribe has instilled in the Abu Hamam survivors a loathing for the Islamic State and the warped brand of Islamist politics for which it stands, said Abu Siraj, another of the tribesmen. A former lawyer, he, like most of the men, asked to be identified only by his nom de guerre because he fears being tracked even to Turkey by the jihadists.

“Now we hate everyone who prays,” he said. “Now we hate even beards.”

But finding support for efforts to organize against the militants is proving hard, he said, pulling out his mobile phone to show a photograph released that day of the trussed, decapitated body of a friend who had purportedly been caught attempting to throw a hand grenade against them.

“When you see your relatives being slaughtered, you will be forced to accept compromises you would otherwise never have been prepared to accept,” he said. “And when you see the world has abandoned you, you will do nothing about it.”

U.S. officials say the Kobane attacks were not intended to show preference for one community over another, but rather served as an opportunity to take aim at the large number of militant fighters who converged on the town to capture it. The Pentagon claims to have killed hundreds of Islamic State militants around Kobane, in keeping with the wider U.S. goal of targeting the Islamists’ infrastructure and resources in Syria to downgrade their ability to reinforce and finance their operations in Iraq.

The primary focus of the American strategy, Gen. Lloyd Austin, the U.S. Central Command leader, stressed last week, remains on Iraq, and on preventing the Islamic State from projecting power there.

“Iraq is our main effort, and it has to be,” he said at a news conference in Washington. “And the things we are doing right now in Syria are being done primarily to shape the conditions in Iraq.”

Such comments have reinforced perceptions among Syrians that the U.S.-led air war does not have their interests at heart. Differences over the purposes and direction of the war risk alienating the many rebel groups that were engaged in battling the Islamic State before the U.S. government intervened, said Steven Heydemann of the U.S. Institute of Peace.

“It’s already become an impediment,” he said. “I don’t think the administration has fully taken on board how much damage the way they’ve conducted this campaign has done to the relationships they’ve developed with some of these actors.”

‘We were finished’

The Sunni areas of Syria occupied by the Islamic State would seem to be a more likely venue for a revolt than Iraq, where the extremists’ extensive territorial gains this year were aided by local Sunni insurgents and tribes alienated by the discriminatory behavior of the Shiite-led Iraqi government.

In Syria, however, the Islamic State’s conquests came at the expense of local rebels who already had fought to eject their government and then found themselves outgunned and outmaneuvered by the newly emerging Islamist extremists.

The Shaitat tribe, along with many others in the oil-rich province of Deir al-Zour bordering Iraq, spent much of this year battling to retain control of their area against encroachments by the Islamic State, and they might have prevailed had the Islamic State not swept into the Iraqi city of Mosul in June, rebels say. The vast amounts of U.S. weaponry the Islamic State captured were trundled across the rapidly dissolving border with Syria, said Abu Salem, who commanded a rebel battalion in the area before he escaped to Turkey.

“After they took Mosul, we were finished,” he said.

Abu Hamam and a cluster of villages nearby were targeted. After the new armaments from Iraq arrived, “we realized we had no hope. We were surrounded. We wanted to save our people,” said Abu Abdullah, another of the Shaitat fighters, describing how they agreed to a truce with the militants in mid-July.

The Islamic State was permitted to enter the town and establish a garrison, but local leaders were left in charge, he said.

Relations quickly frayed. The crunch came, the tribesmen in Reyhanli said, when Islamic State fighters whipped a local man who was caught smoking a cigarette in the street, a crime under the Islamic State’s harsh interpretation of Islam. The man’s brother, incensed, shot at a passing Islamic State patrol, killing one of its fighters.

The brother was arrested and publicly beheaded, triggering an outpouring of rage. Residents marched on the Islamic State’s headquarters, forcing its fighters to flee. The militants then brought in reinforcements and began shelling the town, using artillery they had captured the previous month in Iraq.

After a three-day barrage, the Islamic State militants moved in. They rounded up all the surviving men and boys older than 15 they could find and set about systematically killing them, the fighters in Reyhanli said.

A photo essay on an Islamic State blog boasted of the different ways tribesmen were killed, including beheadings, mass shootings and a crucifixion. A video shows how the militants lined up scores of captives on a road, their hands bound, then set about clumsily decapitating them, one by one. The executioners, speaking in Tunisian, Egyptian and Saudi accents, taunted those not yet dead by swinging severed heads in front of their faces and telling them, “It’s your turn next.”

The tribesmen in Reyhanli, like many other rebel fighters in Deir al-Zour now living in Turkey or elsewhere in Syria, said they managed to slip away using fake identity cards or escape routes honed during their battle against the government.

They said they are plotting their return, to take revenge and fight — without counting on international support.

“We are tribal people. We will never forget to avenge,” said Abu Salem, the commander of the group. “But we will do it by ourselves, in our own way. We won’t take any help from anyone.”