BAGHDAD -- After 2 1/2 years of fighting, US troops are struggling to tame the vast desert border region they call Iraq's Wild West.
American commanders say they are making progress, but acknowledge that in the sprawling, hard-to-patrol area, pushing guerrilla fighters out of one town often means they show up somewhere else, separating and joining up again like beads of mercury.
To the military officers, the problems in western Iraq illustrate the evolving goals of the US mission: They may never decisively defeat the insurgency or seal off the country's borders. Instead, they think in terms of containing the violence and smuggling at a level that Iraqi forces can someday handle, even if that is years away.
''I don't talk in terms of winning," Marine Colonel Stephen W. Davis, who commands the Marines in western Anbar Province, said in a telephone interview. ''Americans like finality. We like to think in terms of a football game -- identify a problem, analyze a problem, solve a problem, and go on to the next problem. That is not the reality of the Middle East. This is like a neverending rugby game."
Western Iraq is the keystone of the country's insurgency and the Sunni Arab discontent that fuels it. In the border region, the cycle of violence and mistrust between Sunnis and the US military plays out on top of a second crucial struggle as US troops try to stem the tide of arms and fighters through porous borders and along the Euphrates River.
Iraq's borders with Syria and Jordan stretch hundreds of miles through the thinly populated deserts of two of the country's most violent provinces, Anbar and Ninevah. Those regions' large, central cities, Ramadi, Fallujah, and Mosul, have long dominated news accounts of fighting in Iraq.
But in the past six months, many of the fiercest battles have been fought closer to the borders, in the smaller towns clustered near the main crossing points into Syria and surrounded by expanses of dry, undulating, and nearly empty land.
About 5,000 Marines are responsible for an area of 30,000 square miles in western Anbar. There, a string of towns along the Euphrates marks an ancient trade and smuggling route that US and Iraqi officials believe is a key pipeline for arms and fighters stretching from the Syrian border, down the river to Ramadi, and then to Baghdad and the rest of the country.
After US troops crushed and occupied the insurgent stronghold of Fallujah in November 2004, military officials say, insurgents fled to the river towns of Husaybah, Karabilah, Ana, Rawa, Haditha, Hit, and Haqlaniyah.
In Ninevah Province, another cluster of towns follows another main road into Syria, and US troops have been fighting bloody battles along that road, centered on the town of Tal Afar, in recent months.
In Anbar, Marines have been launching attacks and sweeps of the Euphrates towns since April, but it has been hard to deal a conclusive blow: Insurgents often flee when troops arrive and return when they leave.
Now, in what Davis calls an evolution of tactics, Marines are working to establish a long-term presence, along with Iraqi forces, in the river towns. Small units are staying in places such as schools and requisitioned houses.
While Marines could fight a pitched battle against thousands of insurgents in Fallujah last year, they now fight on a much more spread-out landscape where fighters melt easily from town to town. They face a political battle, as well, as the fighting spreads resentment among residents caught in the crossfire.
Near the Syrian border, the towns are a jumble of low buildings and large industrial plants idled for lack of maintenance and sabotage. But farther down the river are some of the most beautiful places in Iraq.
Haditha sits at the foot of a hydroelectric dam that holds back a blue lake in a dry reddish landscape that US troops often compare to Lake Powell in the American Southwest. Below it, the towns of Barwana and Haqlaniyah sit among palm groves on either side of a stretch of river that flows clear and blue.
Those towns were relatively peaceful in the first year of the occupation, and foreign reporters could travel there safely. But kidnappings, highway robberies, and attacks on Iraqi forces increased. In a series of skirmishes this spring, insurgents fleeing from Fallujah routed the local army and police along the river.
Now, none of the towns has functioning police forces and the Iraqi army units based there are from other parts of the country, which can fuel resentment among the insular Anbar population.
Northeast and southwest of the river valley stretches desert crisscrossed by gullies and dirt tracks used mainly by Bedouin herders. Seen from a helicopter, the land varies between twisted canyons, flat plains, and low hills, dotted with the occasional tent, pickup truck, and herd of sheep.
US and Iraqi officials say the area provides a last-resort hiding place and travel route for insurgents who are being pushed out by the Marines' recent operations.
''If we can push them to the interior, to those tradition Bedouin areas, then we are well on our way" to containing the insurgency, Davis said.
But there are few instant results, he cautioned. Anbar is one of Iraq's least populous provinces, but the number of US troops killed there -- 698 as of last week, according to a Globe analysis of military figures -- is more than any other province.
Improvised explosive devices, which have caused more than half of all US combat deaths, have become larger and more sophisticated, and some of the deadliest have been used in the river valley. One device in Haditha killed 14 Marines in August in a single armored vehicle.
During operations in Haditha during a two-week period, Davis said, attacks on his forces averaged ''four dozen a day."
Insurgents still wage a powerful intimidation campaign against Iraqis, Davis said, often acting with impunity if US or Iraqi forces are not nearby.
''With 30,000 square miles, you can't be there all the time," he said. ''Nobody owns towns in an insurgent war."
Iraqis used to obeying strongmen as a survival tactic offer little resistance when insurgent come to town, he said.
''You don't need a whole lot of people to roll into a town, beat up people, maybe kill one or two people, and people say, 'OK, we're playing by this guy's rules,' " he said.
Brigadier General Ahmed al-Khafaji, the Iraqi deputy interior minister in charge of border police, said more US and Iraqi troops should be concentrated in the west to further disrupt insurgent movement.
Too many Iraqi soldiers are concentrated in the peaceful Shi'ite Arab south, he said. ''What do we need them in Nasiriyah for? They are just eating and sleeping."
But Khafaji's men lack the basic tools for controlling the borders. His forces man watchtowers spaced about 12 miles apart along the borders, which are not fenced or marked, and are connected only by a rough dirt road. He says he needs surveillance equipment and more roads parallel to the border where patrols could watch for people heading into the country.
Along the border on either side of Husaybah, the main crossing into Syria, there is a 28-mile stretch with no Iraqi border guards, Khafaji said. Marines conducting intense operations there have waved them away, he said.
''They are more afraid for our lives than we are,"' he said. ''We are eager to go there."
A senior US military official based in Anbar, who spoke on condition of anonymity, said Iraqi border posts in near Husaybah had been ''annihilated" in recent fighting. The border has been ignored by migrating tribes for generations, he said. ''It's just a line on a map."
Cultural and linguistic barriers also hamper the effort in Anbar, said a US military foreign-area officer -- a specialist in Arab culture and language -- working there. Very few Marines or interpreters can ''sit down and go back and forth on issues from sewage to trying to identify bad guys," he said.
At the Baghdad taxi stand for people heading to the river valley, many residents of the area still blame Marines more than insurgents for their troubles, saying the sweep carried out before parliamentary elections in January and the constitutional vote earlier this month felt like attacks on the towns rather than the guerrillas.
They described grating everyday difficulties: Schools, public buildings, and roads are often closed; Marines break the windows of locked cars to check for bombs; and people must use boats to cross rivers where bridges are closed by Marines or damaged in fighting.
''The Americans are only harassing the people," said Amal Jassem, 46, who traveled to the capital to pick up food rations that have not reached Haditha for months. She said she moved her family to Haditha early in the occupation to escape violence in Baghdad. But now, she said, she's thinking of moving back.
Globe correspondent Sa'ad al-Izzi contributed to this report.