Iraqi Sheikh Naim al-Gaood was awakened before dawn Thursday, with the grim news that Islamic State fighters had launched a fresh attack on his Sunni tribal area 120 miles northwest of Baghdad.

By nightfall, outmanned and outgunned by Islamic State (IS) forces, and with very little American or Iraqi government support, Mr. Gaood’s Albu Nimr tribe had lost control of 15 villages and seen dozens of its members taken prisoner.

The death of five more Albu Nimr fighters brought the tribe’s death toll this year to 744, he says, which includes some 500 slaughtered by IS in late October and early November.

That massacre was taken to be a clear message to Iraq’s embattled Sunni tribal leaders not to oppose the IS jihadists as they fight back against a growing array of adversaries seeking to undo the lightning gains of spring and summer.

In a final humiliation Thursday, IS sent Gaood a text message: “We will raise our flag upon your noses.”

As Sunni tribes have been forced to choose sides – pro-IS or anti-IS, with many shades of gray in between – new divisions have brought accumulating blood feuds and a scale of slaughter in Anbar Province that is tearing at Iraq’s Sunni social fabric like never before.

Local leaders say IS intimidation is undermining the ability of any tribe to fight back, by using sleeper cells and systematic cleansing of anti-IS figures within the tribe.

The result is that IS is proving much more difficult for the tribes to take on than was Al Qaeda in Iraq (AQI) , whom home-grown Sunni groups fought during the Sunni “Awakening” of 2006-2008 with support from the US.

“The IS considers us agents of the Americans, because we refuse all the terrorists,” says Gaood. “Every day we are bleeding and dying, and need support with weapons. If there is no support we will stop fighting [IS]. We have no equipment for another battle; there is even a shortage of food.”

US Secretary of State John Kerry said last week that more than 1,000 US airstrikes had “halted” IS momentum in Iraq and Syria, where the jihadist group has declared an Islamic caliphate across the one-third of those countries that it controls.

In fact, a pair of US airstrikes on Thursday along the Albu Nimr front killed 10 or so IS fighters, including a commander, says Gaood, and planes are in the air “all the time.”

But it wasn’t enough to save the villages – or the 40 tribe members captured by IS, who Gaood fears will be executed. A call from the office of Iraqi Prime Minister Haidar al-Abadi as the fight raged late morning yielded only “promises.”

After the recent mass killing of Albu Nimr, tribes opposed to IS formed a new alliance to exact revenge, says Sheikh Jabbar al-Fahdawi, a leader of the Albu Fahed tribe. His battle scars include a left wrist that was hit by two bullets in a gunfight with Al Qaeda in 2007, and a limp from a bullet through his right thigh in August, when he battled IS.

“This alliance is an existential battle – a battle to the death,” says Mr. Fahdawi. “The people who are still fighting [IS] lost their sons, fathers, uncles, and cousins, so they keep fighting.

“This [Albu Nimr] killing is a message to the other tribes: If you keep fighting us, we will do the same to your people,” he says. “Even the IS fighters who surrender after this massacre, we kill them, to send a message to IS that we can kill you also. No mercy.”

Since mid-November, he says, IS has lost at least 10 people a day at the hands of the tribal alliance, with bursts of 50, 100, or even 200 dead in specific battles.

On Fahdawi’s mobile phone is a photograph of what he says are the bodies of IS fighters being emptied into a pit from the back of a dump truck. There is also an image of a man with Asian-looking features, shot dead and dusted with dirt.

“We sent him back to China,” Fahdawi says darkly.

The battle against IS is more difficult today compared with the “Awakening” period. Back then most insurgents of AQI (the precursor of today’s IS) were non-Iraqis waging an anti-American jihad, who fought with small arms and roadside bombs.

Their violent actions – including beheading, intimidation, and kidnapping – galvanized local tribes against them, enabling US forces to step in, issues weapons and pay salaries.

But today IS has a vast arsenal of heavy weapons and armored vehicles captured from the Iraqi Army. And among IS ranks are many Sunni Iraqis, who as tribal members themselves understand both the intricacies of Iraq’s tribal system and its military weakness, and are exploiting them.

“Our main weapon is the AK-47, but it is nothing in these battles,” says Gaood, referring to the Soviet-era automatic rifle. “The government doesn’t want to support us, it says weapons will go to IS, but IS doesn’t need them – they already have a much bigger arsenal.”

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Sleeper cells within the tribes have also been active, and have helped IS gather computer databases on some areas under their control. At some checkpoints, tribal leaders say, IS does a quick background check of an individual, looking for past cooperation with the Iraqi government or the Americans.

“They assassinate some people, plant IEDs [roadside bombs], and nobody knew them,” says Fahdawi of the sleeper cells. “But when IS showed up, they began to appear.”