While most insurgents here are not as hardened, that is similar to the numbers in Falluja in 2004, before a bloody Marine offensive to retake the city, said Lt. Col. Scott Jackson, deputy head of the provincial reconstruction team in Diyala, who fought in Falluja.  As the insurgent ranks have swelled, attacks on American troops have soared. The 5,000-member brigade that patrols Diyala Province has had 44 soldiers killed in five months, more than twice the number who died in the preceding year.  On the ground in Baquba, it is not hard to see why. Despite recent seizures of stockpiles, the insurgents have a ready supply of artillery shells and material to make bombs, the biggest killer of American troops here. Some bombs destroy American vehicles. Some are used to booby-trap houses to crash down on Americans. Some are used in larger battle plans: Before overrunning an Iraqi Army outpost south of Baquba, guerrillas laid bombs on the road that Iraqi and American forces would later use to try to rescue the outpost. The minefield blocked the reinforcements, and the Iraqi soldiers at the outpost fled.  The guerrillas seem increasingly well organized and trained. An insurgent force trying to overrun an American outpost in southern Baquba was repelled only after American soldiers fired more than 2,000 Coke-bottle-size rounds from Bradley fighting vehicles and 13,000 rounds from M-240 machine guns.  “They were firing from every direction, trying to get us to concentrate on one spot while the other guys were maneuvering,” said Cpl. Bill McGrath, who said the M-240 barrels glowed cherry red and had to be swapped out a half-dozen times. “These were well-trained military types, not like the guys who shoot tanks with AK-47s. A lot of these guys we never saw. We’d just see muzzle flashes.”  The tactics reflect the skill and resolve of the insurgency here, soldiers say. “To say the guys we are fighting are any less smarter than me, that would be crazy,” said Lt. Col. Morris Goins, commander of the 1-12 Combined Arms Battalion.  The Sunni groups seem to be cooperating like mob families, with ever-shifting alliances. Colonel Goins likens it to the HBO series “The Sopranos.” “We’ll work together today, but when they are no longer of any value,” he said, they part company.  They are capable of disciplined and sustained operations. In early March, a guerrilla force chased a four-man American sniper team through palm groves around the Diyala River for more than two hours, after cutting off the Americans’ escape routes. The snipers were cornered in a sharp bend of the river, officers said, before helicopters finally flew in to rescue them.  Some are purely fanatical. American forces on the main road in western Baquba reported their astonishment during a night in which, over the course of an hour and 15 minutes, they gunned down four teams of guerrillas trying one after the other to plant a bomb in the same spot.  There are many reasons for the mayhem. Diyala and Baquba had significant Shiite and Sunni populations. Shiite-dominated security forces in the city inflamed tensions by persecuting Sunnis, but remain ill prepared to fight the insurgents without support of American forces. Basic government services like food and fuel deliveries have collapsed.  Sunni extremists operate with an extraordinary ruthlessness that terrorizes residents into submission. And Baquba has always had a heavy population of former Baathists and Fedayeen, providing a sympathetic backdrop for the insurgency. Some fighters still wear black Fedayeen uniforms, American officers say.  “Our city has become ruins, even its people,” said one Baquba resident, Mohammed al-Zaidi, 34. “We have no hope to live for.”  Colonel Jackson said he believed that the largest portion of insurgents were disgruntled men or others who just needed money. The rest are homegrown Sunni insurgents, Wahhabis, foreigners and their rivals, Shiite militiamen. Falluja, he said, had a significantly higher proportion of hardened and skilled fighters.  However, he added, “the core of the insurgents in Baquba are as well trained as they were in Falluja.”  Fighters from the Mahdi Army, the Shiite militia largely loyal to Moktada al-Sadr, the anti-American cleric, have also flooded north from Baghdad and now control villages west of Baquba and north of Sadr City. The police chief of Khalis, a city controlled by the Mahdi Army, was arrested by American forces in March for sectarian wrongdoing.  Thousands of Shiites have been killed or displaced in Baquba. But the roots of the gruesome toll that Sunni killers have taken here is partly a consequence of Shiite aggression in Baghdad, where Shiite death squads drove Sunnis out. Many angry Sunnis sought refuge in Baquba, and helped fuel the insurgency.  The human disaster that unfolded in Baquba was a mirror image of much of Baghdad — Sunni death squads wiping out Shiite families. The Shiite-dominated central government in Baghdad responded by sending a new Iraqi Army commander who arrested Sunnis with no evidence, while the recently fired provincial police chief stocked his ranks with Shiite Mahdi militiamen.  American soldiers cited repeated instances of Iraqi troops or police officers terrorizing Sunnis in Diyala. The Iraqi forces’ conduct induced some Sunnis to turn to the insurgency for protection, American officers said. Iraqi lawmakers in Baghdad continue to block provincial elections that would give Sunni Arabs — a majority in Diyala, but one that largely boycotted the last provincial elections — a real stake in government.  As the insurgency has swelled in Baquba, many soldiers here described an American force spread astonishingly thin. The 5,000-member Third Brigade Combat Team of the First Cavalry Division is based in Baquba. But its forces have responsibility over a wide region in Diyala, which is about the size of Maryland, and parts of neighboring Salahuddin Province.  American commanders began a strategy here similar to the new security plan in Baghdad, pushing soldiers into small forward bases deep in insurgent territory.  The troops say that before reinforcements arrived it had essentially been left up to a few dozen foot soldiers and a few tanks from Company B of the 1-12 Battalion to patrol from eastern Baquba to Zaganiya — an insurgent-dominated region of hundreds of thousands of people. “The takeaway was that we had freakin’ next to nothing” for an area with many terrorists, said Capt. Pete Chapman, the company commander.  With areas like Zaganiya receiving little attention, insurgent ranks grew unchecked. Eight of the 300 soldiers in the Fifth Squadron of the 73rd Cavalry Regiment have been killed near Zaganiya since they arrived in March to secure the village. The squadron has been sweeping the area northeast of Baquba, while the Fifth Battalion of the 20th Infantry Regiment rushed north from Taji in March to reinforce Baquba.  A number of officers said additional battalions were still needed for new patrol bases and operations. None would speak for the record. The senior American commander in Diyala, Col. David Sutherland, said he believed there were enough troops in Baquba now.  At one newly built outpost in Baquba, nicknamed Disneyland, soldiers staff lookouts and sniper posts and sleep on cots. They say they control little outside the tall concrete barriers. “You see anybody out there with binoculars, you light them up!” Sgt. Gary Rojas barked on a radio to American snipers one recent afternoon, after an Iraqi insurgent bullet struck the second floor.  Later, Iraqi and American troops walked out of Disneyland, sprinted alongside a wall on the deserted street and then broke into a house 200 yards away. They found the sniper’s nest on the second floor, along with a shell casing. A perfect spot for a sniper, Sergeant Rojas said. The unit climbed into a Bradley to go search another nearby house. First Lt. Karim Branford ordered a move back to the outpost, fearing a trap, before they had gone two blocks from it. “I’m not going to take guys into a baited ambush,” he said.  The Americans said the Iraqis performed well. But the Iraqi soldiers said that most Iraqis assigned to the outpost had fled, kicking back some of their pay to commanders to avoid punishment. Colonel Sutherland said the Iraqi troops were accounted for.  The Iraqi soldiers fretted that the insurgents had better equipment compared with their two clips and rickety Kalashnikov rifles. Like Baquba’s residents, they are intimidated. An Iraqi, Sgt. Raad Rashid, said his countrymen would flee if Americans abandoned the outpost. “Twenty minutes later we’d be gone,” he said. “They would surround this place and kill us.”  The insurgency’s remarkable ability to terrorize residents, killing those who help Americans while coercing others, is undeniably one of its biggest weapons. It appears to be well financed, too.  “Some guys will give you $300 to put this in a hole in the ground and attach a wire,” said John M. Jones, head of the provincial reconstruction team in Diyala, explaining how insurgents recruit bomb emplacers. “Where are the other incentives?”  With the combination of threats and money, Mr. Jones said, the insurgents’ offers are hard for residents to refuse. “You might not agree with the philosophy of what he’s saying, but he’s got the big guns, and they live in the same neighborhood. It’s you, your wife and kids. What can you do?”  Such intimidation makes progress impossible. “We are not able to make even baby steps,” he said. “I hope we’re laying the framework for future baby steps. Right now, I’d say we are pretty much frustrated.”  An Iraqi reporter for The New York Times contributed to this report.  A version of this article appears in print on , on Page A1 of the New York edition with the headline: Attacks Surge as Iraq Militants Overshadow City. Order Reprints| Today's Paper|Subscribe
Baquba has emerged as a magnet for insurgents and, perhaps, the next major headache for the U.S. military.