-- The fighting started in Mosul two days after U.S. tanks entered Fallujah. Armed men appeared in a sudden tide on a main street in Iraq's third-largest city, a wide avenue where so many American convoys had been ambushed that locals nicknamed it "Death Street."

At 11 a.m. Thursday, the target was an armored SUV. Witnesses said that after its Western passengers were chased into a police station, the driver was burned alive atop the vehicle as the attackers shouted "Jew!" The city of 1.8 million people then devolved into chaos. Thousands of police officers abandoned their precinct houses. The governor's house was set alight. Insurgents took the police chief's brother, himself a senior officer, into his front yard and shot him dead.

By Sunday, the dawn of a three-day festival celebrating the end of Ramadan, control over sections of the city remained in doubt. In streets emptied by fear and gunfire, insurgents battled hundreds of Iraqi National Guard reinforcements dispatched by the interim government to quell an uprising that was at once largely expected and disquieting.

U.S. and Iraqi officials said they knew that Ramadan would bring attacks, and that the widely publicized offensive in Fallujah would spark violent provocations in other predominantly Sunni Muslim centers. But the scale of the Mosul attack surprised the U.S. forces in the city. And the disintegration of the city's police force recalled the debacles of April, when a suddenly rampant insurgency shattered faith in the security forces that are expected to assume the ever more difficult task of making Iraq at least reasonably safe.

"They were scaring us, and we are from Mosul, so we withdrew to our houses," said Yusuf Rashid, a police officer in a Mosul neighborhood named "Justice."

As fighting winds down in a Fallujah that has been returned by overwhelming force to the sovereignty of the new Iraq, U.S. forces are turning to the many other cities besieged by a fresh wave of insurgent attacks. The resistance remains concentrated in regions dominated by the Sunni Muslim minority, further complicating the interim government's stated desire to include Iraq's entire population in January elections.

U.S. tanks and attack helicopters on Sunday swooped into Baiji, the midway point between Mosul and Baghdad, where insurgents destroyed a key highway bridge and claimed the city. Masked men carried guns aloft in a protest Sunday in Baqubah, a chronic trouble spot for U.S. forces just northeast of the capital. U.S. forces also engaged fighters in Tall Afar, a largely Turkmen city west of Mosul, and in Hawija, northwest of Baghdad.

Bands of armed men moved freely at night in several neighborhoods of Baghdad, where the number of attacks on U.S. forces has more than doubled from a week ago. Ramadi, 30 miles west of Fallujah, remains a rebel stronghold.

And U.S. and Iraqi forces continue to fight in Samarra, the city advertised as a model for the assault on Fallujah when 1st Infantry Division tanks rolled in there six weeks ago to reclaim the city from insurgents. Under the curfew again in effect there, Samarra residents are allowed on the street for only four hours each morning, and over the weekend its latest police chief, installed just last month, quit.

"We never believed a fight in Fallujah would mean an end to the insurgency," a U.S. Embassy official in Baghdad said. "We've never defined success that way.

"We still have the very difficult problem of a Sunni insurgency."

Just how much the move on Fallujah is roiling the rest of Iraq is a matter still being assessed by Iraqi and U.S. officials. They appear heartened that the country's Shiite majority remains quiescent and largely animated by the prospect of asserting power through the ballot. That marks the sharpest contrast with the April uprising, when militias loyal to Shiite cleric Moqtada Sadr took control of cities across the country's south, opening a vast new military front just as Marines assaulted Fallujah the first time.

Sadr's defeat in August by a U.S. offensive in the holy city of Najaf, followed by weeks of grinding assault in the Baghdad slum named for his father, did much to persuade the radical cleric to shift his energies to politics. For Iraqi and U.S. decision-makers, it also reinforced the decision to confront the Sunni insurgency in its own strongholds.

But if the tactical battle was won in Fallujah -- removing both a symbol of successful resistance and a genuine paramilitary base -- it remains far from clear who will prevail in the larger strategic fight to make the interim government credible to a Sunni population embittered by the loss of influence it enjoyed under the government of former president Saddam Hussein.

The attacks in Mosul did not signal imminent success, at least not to its residents.

"The city is a mess," said Bahaa Aldeen Abdulaziz, owner of the Casablanca Hotel. "The shops are closed. There's no security. And the reason for all this is because the Americans invaded Fallujah.

"And Fallujah will never finish. It has gotten into people's blood."

"I believe the situation will continue like this, and Mosul will become another Fallujah," said Noofel Mohammed Amen, a shoe salesman. "And later on all the cities of Iraq will be Fallujah."

The most immediate concern for the interim government is manpower. Iraq has no more than eight battalions of the newly trained troops, whose main job is to occupy cities after U.S. forces defeat insurgents. Duty in Samarra and Fallujah, which have about half a million people between them, was already stretching that force thin. Adding duty in Mosul "means you're operating right out on the edge of what forces you have -- Iraqi forces," the U.S. official said.

American forces may be stretched thin as well. A battalion deployed outside Fallujah raced back to its Mosul base when insurgents struck, attacking in groups as large as 50 at a time, numbers not previously seen in the city, said Lt. Col. Paul Hastings of Task Force Olympia, the brigade that in February replaced a much larger unit, the 101st Airborne Division.

The magnitude of the Mosul assault generated a wave of excited reports that officials feared would further undermine public order elsewhere in Iraq. The city's governor went on state television to attack "lies" on Arabic-language satellite news channels, which at one point reported that U.S. forces had evacuated one of their main bases. On Sunday, the interim Interior Ministry issued a statement denying that insurgents had overrun two police stations in northern Baghdad.

The news was not all bad for the government. Also Sunday, Najaf buzzed with the news that local tribesmen had carried out three days of devastating attacks in the town of Latifiyah. Located on the exceedingly dangerous road between Baghdad and Najaf, the town harbors extremists blamed for killing 18 young Iraqi men returning from Najaf after signing up for the National Guard earlier this month. The victims' tribal leaders, incensed after extremists demanded payment before handing over the bodies, last week sent fighters north to burn farms and carry out revenge killings, officials in Najaf said.

But in the Sunni Triangle west and north of Baghdad, the insurgency regularly demonstrates its resilience. In Samarra, local insurgents and foreign fighters driven from the city Oct. 1 began trickling back a month later. A wave of car bombs and mortar attacks Nov. 6 killed 17 Iraqi police and made the city a combat zone once more.

Residents assembled each day at the bridge leading from the main highway across the Tigris River into town, shut down by U.S. forces.

"It is our fault," said Abu Muhammed, stranded on the wrong side. "We sold the city to those terrorists and let them enter, and now we cannot enter because of them."

"They made it hard to live till the army came and freed the city," said another man, who gave his name as Abu Omar. "We were able to move around freely and stay out late at night. But now they are back."

Special correspondents Naseer Nouri near Samarra and Saad Sarhan in Najaf contributed to this report.

An Iraqi boy walks past a blazing pipeline near Taji, north of Baghdad. U.S. officials say they expected violence after the Fallujah offensive.Army nurses attend to the body of a Marine who died from wounds suffered in the Fallujah offensive. They had to find another place to store the body, however, as the morgue at the hospital was full.