The invaders pinned notices on the walls of mosques informing residents that they now lived in the Islamic State of Iraq.
For the last year, U.S.-led forces have pursued the militants from one stronghold to the next in Diyala, a province of winding waterways and abundant farms stretching north and east from Baghdad to the Iranian border. They have captured or killed hundreds of people, most said to be members or affiliates of the militant group Al Qaeda in Iraq. The American-led troops have destroyed weapons caches, training bases, bomb-making factories and torture houses.
Yet the Sunni Arab militants identified by many U.S. commanders as their most lethal enemy and the greatest obstacle to stability in Iraq continue to flow into the province and farther north to the regions of Mosul and Kirkuk.
This is not the only place that the militants have established a haven, but the U.S. deems success here as crucial to its efforts to consolidate recent security gains as American troops begin to draw down.
Diyala sits at a strategic crossroads, providing access to Baghdad, Iran and insurgent strongholds in northern Iraq. Its isolated hamlets, thick palm groves and fragrant citrus orchards provide a multitude of hiding places from which the militants unleash gruesome strikes.
Residents say that those who disobeyed the militants were stuffed into cars and brought before religious courts.
"If they don't bring them back in 10 days, that means they are dead," said Ali Jumaa, an aging farmer with a thin mustache, who lives with his wife in a house fronting a canal in Thanira. "They don't return the body."
The U.S. military says Al Qaeda in Iraq is led by foreign fighters. Its Jordanian founder, Abu Musab Zarqawi, was killed in a U.S. airstrike outside Baqubah in 2006. But the military says the foot soldiers are mostly Iraqi, citing detailed ledgers recovered from an insurgent base showing local recruits, one of them just 16.
At first blush, the province would not seem the most obvious place to center a self-styled Islamic caliphate. Unlike the militants' previous stronghold in overwhelmingly Sunni Anbar province, Diyala is a volatile mix of sects, tribes and ethnicities. But the province is also home to thousands of former officers in Saddam Hussein's army, many of whom found themselves without jobs, pensions or a future after the dictator's ouster.
Similar dynamics are found in Mosul, where the Iraqi government has also announced its intention to rout Sunni extremists. With the military push in Diyala province, U.S. and Iraqi officials believe many fighters have fled to Mosul and elsewhere.
"When Al Qaeda got here, they gave them a choice: 'Either you are with us, and we will pay you, or you are against us, and we will kill you,' " said Col. Qais Shahab Ahmed, who commands the police rapid response unit in Muqdadiya, the main commercial center along the Diyala River valley, northeast of the provincial capital, Baqubah.
Local officials say the insurgents paid up to $100 for each tip they received, including ideas about where to hide and information about U.S. and Iraqi troop movements. For the families who supported them, there were also gifts of rice, sugar, and chocolates for the children, villagers said. For those who resisted, retribution was swift and brutal.
When word spread recently that some Sunni and Shiite tribesmen were joining forces with the U.S. military to fight the militants in Baqubah and Muqdadiya, the gunmen began leaving severed heads of those they deemed collaborators along rural roads as a warning to others.
"If you saw the people who cut off people's heads, you would never believe they were capable of this," Ahmed said. "Most are under 18. If you ask them, 'How can you do this?' they will say, 'I don't know. They just gave me a weapon, and I did it.' "
Iraqi officials say the militants are adept at exploiting the poverty, ignorance, resentment and fear in these isolated villages, a patchwork of Sunni and Shiite enclaves.
Sunni residents in the region feel trapped, said Maj. Fuad Farouk, a Sunni who commands a detachment of Iraqi soldiers positioned between the Sunni village of Abu Gharma and Shiite Abu Sayida.
Few Sunnis here trust the Shiite-dominated government security forces, who they say treat Sunnis as terrorists and extort money at checkpoints. So Sunnis were easily lured by Al Qaeda in Iraq's promises to protect them, Farouk said.
Until recently, U.S. soldiers were stationed with Farouk's forces, and he proudly recounts the battles they fought together against insurgent gunmen. But since the Americans handed over the outpost southwest of Muqdadiya to the Iraqis, Farouk says, his soldiers have been hopelessly outmanned and outgunned.
Recently, a young soldier arrived fresh from his wedding. He died the next day in a roadside bombing.
"I felt very, very sorry about what happened," Farouk said, his eyes misting with tears. "He was very happy, because he loved his wife so much. We brought him to his house, and we lay him on his wedding bed."
The militants established their authority in the region by taking control of Sunni mosques, from which they issued decrees.
When U.S. troops pressed into a 50-square-mile area north of Muqdadiya in January in their ongoing offensive, the white-bearded imam in Sinsil Tharia informed them that the senior cleric in the village departed sometime during the last year rather than comply with the militants. Imam Abid Hassim said he replaced his cousin, a moderate cleric who fled when the militants torched his house and wrote on the wall: "Property of the Islamic State of Iraq."
Hassim said notices went up in mosques warning that anyone working for the Iraqi police, army, government or U.S. would be killed. Barber shops, music stores, and coffeehouses were ordered to close. Alcohol and smoking were banned. Women were forced to wear long black robes, with only a slit for their eyes.
Shiites were ordered to leave or were slain. The militants used some of the abandoned homes as safe houses and rented out others to make money.
At least one was turned into a makeshift hospital, according to the U.S. military.
"The people here are poor," Hassim said. "They live a simple life. They have women and children to protect. So they do what the terrorists say."
Since May, U.S. forces in Diyala have uncovered at least six centers where insurgents apparently tortured their victims.
They say that a complex discovered northwest of Muqdadiya had chains attached to the walls and ceiling, bloody tools, a fan belt fashioned into a whip and a metal bed frame attached to a battery that was apparently used to inflict electric shocks. The remains of 26 people were found in communal graves nearby.
To help secure a hold on these tribal communities, some militants sought to marry local women. When three from the Albu Aziz clan refused the marriage demands of Al Qaeda in Iraq emirs, or "princes," gunmen surrounded their village, dragged the women from their homes and slit their throats with jackknives, police reported in December.
Asked where the insurgents came from, residents in village after village said Hembis, one of the larger villages in the area north of Muqdadiya known as the breadbasket of Iraq.
When U.S. forces arrived in Hembis in Stryker armored vehicles, they found a car-bomb-making factory and houses rigged to explode. Hidden on a nearby farm was a recently built base with weapons, maps drawn on the backs of travel posters, a makeshift classroom under the trees and tunnels leading to underground sleeping quarters for three platoons of fighters.
But the militants themselves had vanished. Villagers insisted they could not identify the fighters because they always wore masks. Some whispered there were Saudis, Moroccans, Algerians and other foreigners among them.
The villagers who emerged from behind their gates to stare at the U.S. troops were mostly welcoming, but remained convinced that the masked gunmen would return.
"When you attack one village, they will move to the next. When you attack that one, they will move to the next. You will never catch them all," a despairing Maad Khalaf Khadrish told the U.S. soldiers.
His once prosperous military family was reduced to penury when its businesses in nearby Muqdadiya were destroyed in the fighting and its orchards cleared to make way for a U.S. outpost at Shakarat. The family spent the last of its savings trying to secure the release of Khadrish's kidnapped brother. He was not returned.
But at his remote outpost, in an area not yet targeted by the U.S. offensive, the Iraqi major took a more hopeful stance.
"If we continue with all these operations all the time, they will get weaker and weaker," Farouk said. "And we will destroy them."
Times staff writer Ned Parker in Baghdad contributed to this report.