As the Americans packed up in 2011, they seemed oblivious; the Baghdad government too was dismissive of what in hindsight was a surgically precise operation, launched as early as 2009, to rob the Sunni tribes of the men most likely to lead the charge in a conflict such as that which besets Iraq today.

It says a lot about IS - which campaigns as a force that will "save" Sunnis - that in girding its loins for war with the Shiite-led government in Baghdad, it first set about murdering an entire leadership layer in its own community.

No one took much notice. But Craig Whiteside was counting. A veteran of the Iraq war and now a professor at the US Naval War College, Whiteside has identified 1345  members of the Awakening or Sahwa movement - Sunni tribesmen who fought against al-Qaeda - who have been executed, in what he bills as the most successful assassination campaign since the Viet Cong's attack on the Diem regime in 1959-60.

And he suspects many more were taken down. Writing on the military blog War on the Rocks, he cautions: "This is a massive undercount as the data is based only on confirmed media reports of killings."

Citing Jurf al-Sakhar, a small but strategic town south of Baghdad which forces loyal to the government recently captured from IS control, Whiteside says that 46 Awakening members were reported killed between 2009 and 2013 in 27 different attacks – "most were shot singly or in pairs in the first three years of the campaign; four were sheikhs from the local Janabi tribe".

He explains: "In the Sunni areas where the Iraqi government had little control, it did not take long for the Islamic State to slowly and methodically eliminate resistance, one person at a time."

Although the Awakening in 2007-8, which was bolstered by American money and weapons, was a military success, it also marked a belated US recognition of the role of the tribes in Iraqi society. With the US military's departure, the new Baghdad government short-changed the Sunni fighters, messing with their pay, jailing hundreds of them and reneging on a US-negotiated deal under which they were to be incorporated into the national security apparatus. Instead, then prime minister Nouri al-Maliki perceived the Sunni fighters as a sectarian cell that would work against him.

Then as now, the western province of Anbar was a critical battlefield – but this time around is different.

It's not that Sunni tribes are not fighting – some are. The flaw in Washington's strategic thinking is that not enough are. And in the new Iraq it turns out that just as many of the Sunni tribes refuse to fight for the central government, the central government is reluctant to arm and supply the tribes.

For every area where US military trainers are deployed and weapons and other supplies have been delivered - to Kurdish fighters trying to hold the Syrian border town of Kobane, foir example, and minority Yazidis marooned on Sinjar Mountain in Iraq - there are many Sunni communities in which resentment is rising because there has been no response from Baghdad or Washington to their desperate pleas for help.

The al-Bu Nimr tribe, based in the Hit region of Anbar, believed it would be resupplied as it kept IS at bay, but in October its fighters ran out of ammunition and surrendered – after which IS punished the tribe with a series of massacres in which more than 700 people were murdered in less than three weeks.

Washington can take some solace from the fact that the al-Bu Nimr are undeterred – they remain in the fight despite their losses. But the years of assassinations by IS and the unrelenting brutality of this year's fighting are taking a heavy toll.

In a series ofinterviews, tribal chiefs in Anbar complained that fear had driven many from the region, including their fellow chiefs; and that pleas by those who remained, to both Baghdad and Washington, for weapons, ammunition and other supplies had elicited little or no response.

Sheikh Naim al-Gaood, of the al-Bu Nimr, claimed the tribes were taking territory back from IS, but complained that they had neither the weapons nor the technology to match IS in battle – "Baghdad keeps promising to supply us, but their promises are not sincere".

"Maybe 96 per cent of our tribe is with Baghdad; but overall, about 20 per cent of the tribes are with IS and another 10 to 15 per cent are standing back, refusing to take a side," Sheikh Naim said.

A major problem for Sheikh Rafa Abdul Karim, of the al-Bu Alwan tribe in Ramadi, is in the mass exodus from the region by tribesmen and their leaders. He told Fairfax Media: "The majority of those remaining are with the government, but we're not winning this fight because we have no weapons and no money.

"The government worries only about itself and its own people, so there's a deliberate delay in supplying us – even the police don't have the weapons they need."

Citing the deaths of 739 members of his tribe in mass killings by IS, he said: "The main reason we have been attacked is because in 2003-04, we were seen as friends of the US. We refused to let the insurgents put roadside bombs in our area, so we were blacklisted from the start – and we have had no one to protect us since the US fled."

Another al-Bu Alwan chief in Ramadi, Arif Mukhaibar, complained that the war in the west of Iraq was a stalemate. "We are winning and losing in an even game," he said. "I have no idea why the US is so gentle in its air strikes – it's a puzzle for me."

Complaining about IS' elimination of the veterans of the 2007-08 Awakening campaign, he said: "The sheikhs are the highest figures in the tribes – it's a huge deal when they are murdered. But there is another problem for those who stay because those who are in exile don't fully understand what is happening here, but they're the ones the Americans talk to. They're always talking to the wrong people."

Asked if they war could be won without dramatic improvements to aid from Baghdad and Washington, he went on: "It's going to be very difficult – we have the fighters and the spirit, but we don't have the weapons and we don't understand why the Americans can't see that.

"They have the slogan 'war on terror,' but they won't fight it with us."

Speaking from Fallujah, closer to Baghdad, Sheikh Mohammad Ramadi of the al-Bajari tribe argued that Baghdad was not winning the war because there was little sympathy among Sunnis for the Shiite-controlled government.

"We have rights that Baghdad could fulfill in 24 hours, but it refuses," he said. "We're so sick of this government and of the Iraqi Army, that people are ready to fight with IS because of the loss of their family members and because of damage to their property."

Explaining the current lack of tribal resolve compared with 2007-08, he explained: "It was very much an American agenda during the Awakening – they gave the tribes money. It was all about politics and money, but the people don't believe that any more.

"Why would they fight for a government that refuses to respect them? [New Prime Minister Haider] Abadi is just another face on the same Shiite agenda – and the Sunni anger will continue until there is meaningful change in Baghdad."

Apart from the absence of the assassinated tribal fighters, an analysis by the Carnegie Endowment for International Peace warns that other changes in the years since the Awakening have altered the terrain – the US does not have the on-the-ground presence it had in 2007 to cut unilateral deals with the tribes, and the Baghdad government on which it relies is deeply sectarian and has insufficient reach into the provinces, say analysts Frederic Wehrey and Ala Alrababah.

At the same time, they write, IS has proved more entrenched and adaptable than its predecessor, as it unleashes a "potent mix of extreme violence and soft power to both coerce and co-opt the tribes".

Despite the election of a new parliament and the appointment of Mr Abadi, they argue that little has changed in Baghdad apart from the rhetoric: "[Mr Abadi] took several steps to accommodate the tribes and to temper the IS' heavy-handed military campaign in Sunni areas [but] on balance, the Iraqi government's support for the tribes has fallen far short of what would be required to turn the tide against the IS in [Anbar]."

Some key tribal figures also refuse to believe that Mr Maliki, who stood aside at Washington and Tehran's insistence so Mr Abadi could take over, has really been neutralised politically – or that Mr Abadi can emerge from his predecessor's long shadow.

Mr Maliki continues to have formal influence both as a vice-president of the country and as head of the Dawa Party, of which Mr Abadi is a member. Mr Maliki also has a powerful personal network – when Mr Abadi sacked 85 former Maliki employees from the Green Zone, including Mr Maliki's son, the former prime minister immediately recruited them to the payroll at the Office of the Presidency.

In the crude politics of the desert, tribal support for the so-called Islamic State is a negotiation with Baghdad – an opening bid that effectively states "give us a better deal and we'll consider switching sides".

"It's important to note that in the historical memory of Sunni tribes, the recent violence by the Islamic State is simply the lesser of two evils," the Carnegie Endowment analysts write. "The brutality meted upon the tribes by the Iraqi government is no better and often perceived as worse because of its sectarian character."

Or as Sheikh Zaydan al-Juburi told the two researchers: "We choose [IS] for one reason – [IS] only kills you; the Iraqi government kills you and rapes your women."