Spc. Jeremiah Westerfeld, left, 22, Batesville, Ind., and Sgt. William Rose, 26, Arlington, Mass., stake out positions at an abandoned school in Baqouba, Iraq. They are members of the U.S. Army's 5th Battalion, 20th Infantry Regiment, 3rd Brigade, 2nd Infantry Division, who faced fierce battles on their first day of patrols in Diyala province.

Baqouba, Iraq ? Dozens of U.S. Stryker combat vehicles roared into Baqouba at sunrise. The enemy was ready. As the dawn call-to-prayer fell silent, the streets blazed with insurgent fire.

Within minutes of the start of their first mission Wednesday in Diyala province a voice crackled across the radio: “Catastrophic kill, with casualties.”

Inside the rear of one Stryker, soldiers shushed one another and leaned closer to the radio. They all knew what it meant. A U.S. vehicle had been lost to hostile fire.

Nearly 100 Strykers, armored troop carriers with 50-caliber machine guns, were called north from Baghdad into the province and its capital to try – yet again – to rout Sunni insurgents, many who recently fled the month-old Baghdad security operation.

The fighters have renewed their campaign of bombings and killings just 35 miles northeast of the capital as the war enters its fifth year. Diyala province is quickly becoming as dangerous as Anbar province, the Sunni insurgent bastion west of Baghdad.

Rocket-propelled grenades pounded buildings Wednesday where U.S. soldiers sought cover. Mortar rounds soared overhead and crashed to earth spewing clouds of deadly shrapnel.

Gunfire rattled ceaselessly – the hollow pop of insurgent AK-47s and whoosh of grenade launchers nearly drowned out by shuddering blasts from the 50-caliber machine guns.

Soldiers screamed into their radios for backup. Apache attack helicopters swooped in, firing Hellfire missiles.

By day’s end, one soldier was dead, 12 wounded and two Strykers destroyed. The Americans said dozens of insurgents were killed but gave no specific number.

It was a bloody first day for the 2nd Infantry Division’s 5th Battalion, 20th Infantry Regiment – the crack Stryker battalion dispatched from Baghdad’s northern suburbs.

“They threw everything at us – RPGs, mortars – and a guy even tossed a grenade just in front of my vehicle,” said Capt. Huber Parsons, the 28-year-old commander of the 5-20’s Attack company. “But the most devastating was the IEDs,” the Coral Gables, Fla., native said. He was talking about improvised explosive devices – roadside bombs.

One Stryker was lost in a particularly sophisticated ambush.

Struck head-on by an IED, the rubber-tired armored vehicle was swallowed up in the bomb crater. Insurgents emerged from hiding, firing RPGs in unison.

The Stryker crew was trapped. One U.S. soldier was killed. All nine other crew members were wounded, though six later returned to duty.

The other Stryker was destroyed when a roadside bomb exploded as the armored vehicle drove over it. The nine-man squad got out alive, three with injuries.

“It was quite an introduction to Diyala,” said Sgt. William Rose of the 5-20’s 3rd platoon, Alpha company. “That was the most contact we’ve had in weeks, maybe months,” said Rose, a 26-year-old Arlington, Mass., native.

“They always say the next place we’re going is the worst – the most violent – and it never turns out to be the case,” Rose said. “They really meant it this time.”

Violence has risen dramatically in Diyala since the Feb. 14 launch of the Baghdad security operation. Insurgents have been taking control gradually for months, however. Attacks on American forces in the province have shot up 70 percent since July, according to military figures.

The Stryker group sent to fight the insurgents was hand-picked by Gen. Ray Odierno, the second in command of all U.S. forces in Iraq. It marked the opening of a new front in the Baghdad security operation, a broadening of the mission for which President Bush has promised more than 20,000 additional soldiers.