MOSUL, Iraq—It’s not a pretty sight: Sagging skeletons of two- and three-story buildings under a threatening gray sky. Abandoned shops with corrugated iron fronts riddled by bullet holes. And amid the garbage heaps and pools of fetid rainwater, a roadside bomb set to explode.

Five years after the U.S.-led invasion and following a significant drop in violence nationwide over the past year, the battle for Mosul, Iraq’s third largest city, still waxes and wanes.

“This is our hottest area,” says Sgt. 1st Class Ron Corella, a decorated combat veteran in this war-scarred quarter of the ancient city where moments before his troops spotted — and disarmed — that roadside bomb.

“The enemy knows that if we gain a foothold and they can’t push us out, it’s another safe haven they have lost. So they have to fight,” Corella added.

Lt. Col. Robert Molinari, executive officer of the 3rd Armored Cavalry Regiment, says Mosul “looks like Baghdad about 18 months ago” at the height of violence in the Iraqi capital.

It was the generally successful pacification of Baghdad — the fruit of the so-called troop surge — that drew al-Qaida and other insurgents to this hub of northern Iraq to open a new battleground and safeguard their infiltration and supply routes.

But on-off security clampdowns, a lack of aid money and a power struggle between Kurds and Sunni Arabs are also blamed for Mosul’s woes.

In the city’s version of the Baghdad surge, 22,000 U.S. forces and Iraqi troops and police have spread out in an operation called “Mother of Two Springs” — taken from an Arab nickname for Mosul — that began in May and went into a new phase Oct. 15.

Armored vehicles snake through mile-long lines of traffic, backed up behind checkpoints. Soldiers man sandbagged positions atop houses and mosques. Iraqi and U.S. troops stage patrols around the clock from some 40 makeshift bases in the city of 1.8 million people.

U.S. and Iraqi commanders cite some progress after months of struggling to root out insurgents in street-by-street battles.

Attacks, they say, are down to fewer than 70 a week, compared to about 130 before May. Insurgents have had to switch from well-coordinated attacks to hit-and-run strikes and hurried planting of roadside bombs. Some city quarters are relatively safe, with commerce reviving and shops staying open after dark.

“The people feel more secure, so some dare to come forward with tips about the bad guys,” says police Lt. Col. Adel Kader, hunkered down in the Hadba district, one of the city’s most violence-ridden.

But nobody is yet declaring victory, and Molinari says the current military push “just treats the symptoms, not the problems” of ethnic politics and a wrecked economy.

On the security front alone, Mosul is a complex nut to crack. Not just al-Qaida, but more than a dozen Sunni Muslim and other insurgent groups are on the loose, together with criminal syndicates and rival tribes.

“Al-Qaida looks to Mosul as a gateway to Iraq. It’s a place that it doesn’t want to lose,” says Molinari, from Fort Hood, Texas. “It is not as such the last stand of al-Qaida. It’s a last stand to maintain their lines of communication, thus their viability to conduct operations in Iraq.”

Roughly equidistant from the borders of Syria, Iran and Turkey, Mosul has been an important junction on trade and smuggling routes for centuries. The route from Syria across the desert and along the Tigris River is the prime conduit for fresh insurgents.

The core of the insurgency is in the Ottoman Empire-era old town and nearby western Mosul — densely populated areas, interlaced by narrow alleys stacked with cheek-by-jowl houses and burrowed under with tunnels and caverns.