BAGHDAD (Reuters) - Ahmed froze as he opened the small white envelope left on his doorstep in the Iraqi town of Latifiya. Shaking, he looked around before reading the words scrawled on the envelope. Inside was a bullet.
"The message was clear: I must leave or I will be slaughtered," said Ahmed, who immediately left home with his family and is now living with relatives in another town.
Ahmed, who did not want his full name used, was targeted for belonging to a government-backed Sunni Muslim militia formed at the height of Iraq's sectarian conflict in late 2006, when Sunni tribesmen joined forces with U.S. troops and rebelled against al Qaeda in what came to be known as the "Sahwa" (Awakening).
But the tide is now turning back toward al Qaeda and other Islamist insurgents whose onslaught against the Shi'ite-led government and its allies has killed more than 6,000 people this year in an ominous echo of the bloodshed that peaked in 2006-07.
Iraqi security officials blame the surge in violence partly on a lack of cooperation from Sahwa fighters who feel they were not rewarded as promised for taking on al Qaeda earlier and have been left to face the backlash from the militants alone.
"Since 2006, we have fought al Qaeda and arrested so many of those criminals but today we are going to back to square one," said Sheikh Aref al-Jumaili, a tribal leader from a town in Anbar province, Iraq's Sunni heartland.
"We cannot fight them now. They will kill us and get revenge because we fought them with American support. Today this government is not able to protect or support us."
In their heyday, the Sahwa mustered around 103,000 men, but the number has declined to no more than 38,000 since the U.S. military relinquished security control in Iraq in 2010, according to Prime Minister Nouri al-Maliki's reconciliation adviser Amir al-Khuzaie.
Some were hired as civil servants, integrated into the ministries of defense and interior or given early retirement.
"Sahwa (fighters) have left a big gap after they abandoned their positions and stopped securing their areas," said a senior military officer serving in the Sunni town of Shirqat, around 300 km (190 miles) north of Baghdad.
"They know exactly the areas where al Qaeda and other militants are operating, how to abort their attacks, chase and hunt them," the officer said.
In Sunni communities, the Sahwa once provided intelligence in areas where the armed forces, staffed mostly by Iraq's majority Shi'ites, face mistrust if not outright hostility.
But Sahwa fighters themselves now face the ire of fellow Sunnis as resentment builds towards the Shi'ite-led government that came to power following Saddam Hussein's overthrow in 2003.
Sunnis took to the streets in December in protest against Maliki, a Shi'ite, seeing his pursuit of Sunni politicians on terrorism charges as part of a pattern of oppression.
A deadly raid by security forces on a protest camp in April touched off a violent backlash by Sunni militants who view Shi'ites as non-believers and the Sahwa as "Sunni apostates" who deserve to die for betraying God and their sect.
"We will kill them in a brutal way and throw their corpses to the dogs," read a recent statement posted on the Internet and signed by al Qaeda's local affiliate, the Islamic State of Iraq.
Security officials said Sahwa fighters and their families had come under frequent attack in the past six months, but could not say exactly how many had been killed.
In northern Iraq, where insurgents have a foothold, al Qaeda gave the Sahwa an ultimatum that expired this week on the first day of Eid al-Adha, the Muslim Feast of Sacrifice, to repent and swear allegiance to the Islamic State of Iraq or face death.
Al Qaeda's Iraqi wing has told Sahwa fighters to hand over their weapons and uniforms and to record a video pledging their allegiance to the militant group on a flash memory stick, Sahwa leaders and security officials said.
A senior military intelligence officer said the aim was to force former Sahwa members to fight alongside al Qaeda or face the prospect of these videos being sent to the security forces.
"We do not trust al Qaeda, but our fear (of it) forced us not to cooperate with the government," said Ahmed, adding he would rather spend time in jail than join the militants. "Al Qaeda for me is like a nuclear bomb, it is a source of death".
Al Qaeda's resurgence in Iraq has been nourished by the civil war in neighboring Syria, which has drawn Sunni Islamists from across the region and beyond into battle against President Bashar al-Assad, whose Alawite sect derives from Shi'ite Islam.
Al Qaeda's Syrian and Iraqi wings merged earlier this year to form the Islamic State of Iraq and the Levant, which has claimed responsibility for attacks on both sides of the border.
Facing this invigorated Sunni insurgency, Iraqi security officials said Maliki had decided to revive the Sahwa project. In Syria, some concerned by al Qaeda's growing clout are also looking to replicate the model there.
However, one Sunni politician, who asked not to be named, said Maliki had in fact undermined the Sahwa by encouraging divisions among Sunni tribal leaders in order to control them.
"It's a great way to create a strife among them and push them from afar to fight each other," he said.
(Additional reporting by Kamal Naama; Writing by Suadad al-Salhy; Editing by Isabel Coles and Alistair Lyon)