In the soft heat of the late evening in this southern Iraqi town, men come to the Najmawi coffee shop to play dominoes. Lights illuminate faces. Tiles click quietly.

The scene, which unfolds nightly in a garden near the Tigris River, is unimaginable in central and western Iraq, where night turns neighborhoods into dangerous ghost towns.

It was not always this tranquil. It used to be that Shiite Islamists smashed furniture and stopped games at Najmawi's. Dominoes, they said, were un-Islamic.

The British forces charged with security here did not intervene. They have enough troubles of their own, they said, trying to keep themselves safe on routine patrols.

But in a surprising twist, the region's tribes, particularly powerful in neighborhood life here, have weighed in forcefully, using their authority, often in a nonviolent way, to force the Islamists to back off.

Large portions of Iraq's rural south have been relatively peaceful, like the provinces of Muthana, where Japanese forces recently left, and Dhi Qar, whose capital, Nasiriya, is patrolled by the Italians, and it is possible that tribes are playing a similarly quiet but central role in keeping order.

"Now people feel safe," said Hassan al-Najmawi, 31, the coffee shop owner. The Islamists, he said, "have lifted their hands from the dominoes, the music shops, the young people's clothing."

The story of how Amara, the capital of a leaf-shaped province called Maysan, came to be relatively safe for its citizens — even as danger increased for the British — is a hopeful tale of small-town camaraderie, fierce independence and, above all, tribal power.

It offers an alternative to the grim formula that has applied in the provinces where a poisonous mix of sects, political parties and ethnicities has led to relentless bloodletting that the central government has been powerless to stop.

The differences reflect, in part, fundamental splits between rural and urban life in Iraq. Maysan, a province of about 920,000, is the countryside. More than 60 percent of its work force is employed in the private sector, mostly farming; in the wealthier, urban areas, a majority is employed in public service.

Because it is rural, it is smaller and easier to control than the bigger, more fractured communities in the cities, like Basra to the south, Iraq's second largest city.

There, many political parties and their militias are fighting over control of the province and its oil. Amara has just two militias, both Shiite: the Mahdi Army, loyal to the anti-American cleric Moktada al-Sadr, and the Badr Organization, the armed wing of the Supreme Council for Islamic Revolution in Iraq.

But the most important factor is the network of tribes.

In cities, generations of busy urban life have dulled people's tribal connections, while in the countryside, particularly in the south, tribes oversee all aspects of daily life — celebrating weddings, intervening in family disputes, administering justice after a killing and collecting money to help someone in need.

The 14 or so tribes in Maysan, with its vast expanses of marshes, are particularly strong — if often at odds with one another.

Even Saddam Hussein could not force the marsh Arabs, as they are known, to submit. He resorted to digging giant canals to drain their wetlands and stamp out their way of life.

"The tribe in Iraq is the basic building block of society," said Abd al-Kareem al-Mahamedawy, a tribal sheik from Amara who fought Mr. Hussein for years.

Mr. Najmawi is a smiling 31-year-old who laughs a lot and has difficulty sitting still. He has lived in Amara his whole life, and took it personally when, about a year after the American invasion, religious Shiites and their militias began to boss people around. Local men who called themselves members of the Mahdi Army, the grass-roots armed group that follows Mr. Sadr, began to come to his shop and disrupt business.

"They are troublemakers," he recalled, sitting in an office in his shop on a balmy evening in late May. "They don't feel shy. They do anything." The militia asked to use his cafe for a meeting. He refused. He later allowed a civil society group to hold a seminar on corruption in Amara. His shop was bombed that night. Suspicions focused on the Mahdi militia.

Meanwhile, the Islamists were using the new lawlessness to impose their will, delivering lashes to those found with alcohol in their cars and confiscating un-Islamic merchandise. But the population of Amara is small, about 300,000, and residents knew who was part of the militia.

"I have a big coffee shop," Mr. Najmawi said. "You see how many people are in it. I know who likes Mahdi militia, and I know who hates them. Let me tell you, only a very few join them."

The reaction of the tribes was hardly a foregone conclusion. In the American-led effort in Iraq, they were an afterthought. Few were courted, and they were all but left out of the political process, which began here in earnest in early 2005.

Followers of Mr. Sadr dominate the 42-member provincial council. The governor is a former Mahdi Army commander. The police chief is a former Badr member.

"We didn't go the extra mile to encourage the tribes to participate, which was perhaps a mistake," said Edward Ferguson, the British political adviser in Amara.

And they fight among themselves. Lt. Col. David H. Labouchere, the commander of the British forces here, described what appeared to be tribal fighting that he watched at night from a helicopter in April. Mortars and rocket-propelled grenades exploded without stop, and when the electricity went out, the village, outside of Amara, "lit some fires and kept on fighting," he said.

But a direct attack from a militia brought the tribes together. In May 2005, two Mahdi fighters bombed the Manar engineering company, which belonged to the Kaabi tribe, infuriating its leaders. In a tribal trial called a fahsal, the attackers' families agreed to pay $18,000 in damages and to banish the two men from Amara.

A short time later, the Kaabi tribe invited the town's leading tribal sheiks to its mudhif, a guesthouse made of reeds that is the rough equivalent of a town hall.

"We invited them all," said Khalid Jabor al-Ali, the Kaabi sheik.

The gathering agreed to punish any militia member who attacked a tribe or its property. The tribes would watch the townspeople and the known militia members among them.

Suddenly, the stakes for carrying out an attack were substantially higher.

"We said anyone who does rebel attacks, his blood will no longer be tribal blood, and his tribe will reject him," Mr. Ali said, sitting on a mat in the same mudhif.

Gradually, quietly, he and other sheiks said, life began to shift.

Amar al-Sadi, 24, the owner of Al Qawthar Music Shop, said attacks on shops, common a year ago, had become rare. Islamic militia members still come from time to time to make sure the shop, which also sells movies, has no pornography, but mostly he is left alone.

Even people on the street seem irreverent. On an evening in late May, a group of young men stood just half a block from Mr. Sadr's central office in Amara, clapping and singing. A woman walked past a cleric and made a gesture with her hands, as if shoving him away, the Iraqi equivalent of a raised middle finger.

"The clerics are pushing the people, and that makes the people hate them," said Kadham al-Mashetet, the sheik of the Sarrai tribe, who saw the angry gesture.

Late last year, the number of killings began to drop. In March of this year, the police in Amara registered six homicides, the lowest rate since the British military began collecting data in the beginning of 2005. The average rate for the first five months of this year fell by 71 percent compared with the previous five months, according to British military figures.

The trend is the opposite in Basra, where violence has risen significantly in the past year. With the tribes watching in Amara, the stakes for killing were higher, and militia members weighed attacks carefully. The militias themselves seemed intent on preserving the status quo.

The police chief, Ismail Arar Qadim al-Majdi, linked to Badr, said he tried to avoid retaliatory attacks.

"I know who is doing the killing here," Mr. Majdi said in an interview. "When I arrest someone, they reply by a mortar. If I keep up this arresting, it will be the same as in Basra. I want to keep the city quiet, so I try to avoid this."

It is not clear, however, how firm his control is. Late last month, the son of a sheik was killed in a shooting, an ominous crime that has baffled the town. Some sheiks blamed Iran, where the Badr Organization had operated in exile. For the British soldiers here, the power of tribes can be vexing, particularly during efforts to build a Western-style legal system. Six months ago, for example, a man shot another dead in front of a police officer. He was arrested, taken to jail and then promptly released, after the attacker's tribe agreed to pay $3,000 and promise three women in marriage to the family of the dead man, said Colonel Labouchere, the British commander.

"Tribes hold the key to this province," he said. "If we try to employ our own version of law in this country, we are not going to win."

The tribes have helped the occupying forces at times. They successfully mediated in 2004, when the Mahdi militia fought the British in an uprising across Iraq's south inspired by Mr. Sadr.

At that time "the tribes went to the Sadr office and said, 'Take your guys off the streets,' " said Mr. Ferguson, the British political adviser. In Dhi Qar Province, west of Amara, tribal leaders recently interceded in a similar fashion, he said.

The role of the tribes in the American-led effort may continue to grow.

Colonel Labouchere, who cares for two pet goats in a sleepy outpost in a former Hussein military camp and is planning the British withdrawal from the area, said he hoped the tribes could replace his forces as the central pillar between the Madhi Army and the Badr Organization.

In an effort to exert more influence on politics, about 1,000 tribesmen gathered in March and established a new provincewide tribal council of 35 sheiks.

And if Iraq holds a new set of provincial elections as scheduled for later this year, the tribes might increase their direct representation.

For now, Mr. Najmawi takes pleasure in testing the Islamists.

In a bold affront, he decided to show the World Cup games in his garden on a big screen for maximum effect. He even advertised around town with posters.

Before the first game, two men from Mr. Sadr's office came and delivered a warning: Go to the Sadr office and obtain permission. Soccer, they said, is un-Islamic.

"I never went," Mr. Najmawi said.

Since then, his shop has been packed with more than 500 customers a night, and the only violence has been the breaking of several chairs.

"I cannot let them control me," he said. "They are not in charge here."

Copyright 2006 The New York Times Company