BAGHDAD — A string of heavy losses from powerful roadside bombs has raised new questions about the vulnerability of the Stryker, the Army's troop-carrying vehicle hailed by supporters as the key to a leaner, more mobile force.

A Schofield Stryker brigade — 3,900 to 4,000 men and women — will deploy to Iraq later this year, possibly for as long as 15 months.

The announcement represents an increased commitment of Hawai'i-based troops to the war in Iraq and the first deployment for Schofield Barracks' new Stryker brigade.

The notification to the 2nd Brigade, 25th Infantry Division was part of an announcement that more than 35,000 soldiers Armywide will deploy to Iraq beginning this fall.

The exact date hasn't been set yet, but Schofield soldiers have said they will ship out to the National Training Center at California's Fort Irwin in July or August, then possibly deploy to Iraq in December.

Schofield Barracks has two infantry brigades of 3,500 to 4,000 soldiers, and the upcoming deployment for the 2nd Brigade — which converted to a Stryker unit — is the second to Iraq since 2004.

The 3rd Brigade at Schofield deployed to Afghanistan in 2004, and along with the 25th Infantry Division headquarters and aviation brigade — a total of more than 7,000 soldiers — left for a year of duty in northern Iraq last July and August. That deployment has since been extended to 15 months.

The Stryker vehicles provide greater protection than a Humvee but are not invincible. The 19-ton vehicles can stop a 14.5mm heavy machine-gun round. With an additional 5,000 pounds of birdcage-like "slat" armor, Strykers also can withstand attacks from rocket-propelled grenades.

However, U.S. officials said that since the Strykers went into action in violent Diyala province north of Baghdad two months ago, losses of the vehicles have been rising steadily.

LOSSES ARE CLASSIFIED

A company in Diyala lost five Strykers this month in less than a week, according to soldiers familiar with the losses, who spoke on condition of anonymity because they are not authorized to release information. The overall number of Strykers lost recently is classified.

In one of the biggest hits, six U.S. soldiers and a journalist were killed when a huge bomb exploded beneath their Stryker on May 6. It was the biggest one-day loss for the battalion in more than two years.

"We went for several months with no losses and were very proud of that," a senior Army official said in Washington, speaking on condition of anonymity because he is not authorized to comment publicly. "Since then, there have been quite a few Stryker losses."

"They are learning how to defeat them," the Army official said of Iraqi insurgents.

The military introduced the eight-wheeled Stryker in 1999 as the cornerstone of a ground force of the future — hoping to create faster armored units more agile than tank units, but with more firepower and protection than light-infantry units. The Army has ordered nearly 2,900 vehicles for its $13 billion Stryker program.

But the Army and the Marines are already looking for something different that can survive big roadside bombs — the main threat to soldiers in Iraq — meaning the Stryker's high-profile status as the Army's "next-generation" vehicle may be short-lived.

"It is indeed an open question if the Stryker is right for this type of warfare," said Michael O'Hanlon, a senior analyst with the Brookings Institution. "I am inclined to think that the concept works better for peacekeeping. But based on data the Army has made available to date, it's hard to be sure."

MOBILITY IS KEY

Supporters of the Strykers, which have been used in Iraq since late 2003, say the vehicles that carry two crew members and 11 infantrymen offer mobility, firepower and comfort.

Lighter and faster than tracked vehicles like tanks, each Stryker can rush soldiers quickly to a fight, enabling commanders to maintain security over a wide area with relatively fewer troops. Humvees can carry only four soldiers — and are more vulnerable to bombs even when their armor is upgraded.

"I love Strykers," said Spc. Christopher Hagen, based in Baqouba. "With Strykers, you're mobile, you're fast. You can get anywhere anytime. They bring a lot of troops to the fight."

But some analysts have long questioned the wisdom of moving away from more heavily armored tracked vehicles, like tanks and Bradley fighting vehicles, to wheeled transports, like the Stryker.

They say that is especially true in Iraq, where powerful bombs — not rocket-propelled grenades or small-arms fire — are the main threat.

"The Stryker vehicle was conceived at a time when the Army was more concerned about mobility and agility than it was about protection," said Loren Thompson, a military analyst from the Lexington Institute. "Stryker was the answer to that need."

The Stryker's vulnerabilities have become increasingly apparent since a battalion of about 700 soldiers and nearly 100 Stryker vehicles from the Army's 2nd Infantry Division was sent to Diyala province in March to bolster an infantry brigade struggling to restore order there.

Trouble started as soon as the Strykers arrived in Baqouba, the provincial capital of Diyala.

U.S. commanders ordered the vehicles into Baqouba's streets at dawn the day after they arrived. The hope was that the large, menacing vehicles — each armed with a heavy machine gun and a 105mm cannon — would intimidate insurgents and reassure local residents.

Instead, insurgents hammered the Strykers with automatic weapons fire, rocket-propelled grenades and a network of roadside bombs. By the end of that first day, one American soldier was dead, 12 were wounded and two Strykers were destroyed.

A few days before the May 6 attack that killed the six soldiers and a Russian journalist, troops scrambled out of another damaged Stryker and took cover in a house while they watched the vehicle burn. Several of them were injured, but none seriously.

MORE POTENT BOMBS

Losses have since mounted.

A few days after the May 6 blast, two Strykers were hit by bombs, and one soldier was killed and another seriously wounded.

Lt. Col. Bruce Antonio, who commands a Stryker battalion in Diyala, said he and soldiers still have confidence in the Strykers and noted they had survived many bombs, which the military calls improvised explosive device or IEDs.

But Antonio said some insurgents had found "the right mix of explosives and IED positioning to inflict severe damage on the vehicle." He also noted that tanks had proved vulnerable as well.

The insurgents also apparently are becoming better at hiding the devices — the IED that killed the six soldiers and the journalist was believed hidden in a sewer line. To add potency, insurgents surrounded the device with cement to channel the blast force up into the vehicle, soldiers familiar with the inquiry said.

Supporters of the Strykers say all that proves that it's the lethality of bombs in Iraq — not the Strykers themselves — that are the problem: The bombs are now so powerful that even Abrams main battle tanks are vulnerable to some of them.

"I'm not sure if it's any reflection on the (Stryker) but rather on how things are getting worse" in Iraq, according to a senior Democratic congressional staffer who tracks Army programs, who spoke on condition of anonymity because he wasn't authorized to speak publicly.

MINE RESISTANT VEHICLE

Stryker soldiers said that when they were based in Mosul in the north, roadside bombs were often little more than pipe bombs. In Baqouba, the bombs are bigger and buried deeper, making them difficult to detect.

"With what we got hit with the other day, it wouldn't have mattered what we were in," said Spc. John Pearce, speaking of the May 6 bomb. "We were going to take casualties, regardless."

Either way, the Army and Marine Corps already are pushing for new Mine Resistant Ambush Protected vehicles, or MRAPS, whose V-shaped hulls are designed to deflect bomb blasts outward, rather than through the vehicle.

The Pentagon has requested nearly 7,800 of the new vehicles at a cost of $8.4 billion and is considering ordering thousands more to give soldiers better protection.

Such moves, however, serve only to reinforce the views of critics, who say the Army opted for a vehicle that was useful in Balkan peacekeeping or other "low-threat" missions but is inadequate in so-called "asymmetric warfare," where a weaker opponent devises simple tools to exploit a strong opponent's weak points.

"As long as the Stryker-equipped light infantry was used ... against lightly armed insurgents, there was no problem," said retired Col. Douglas Macgregor.

"Now, they are being tossed into the urban battle, where only tracked armor can survive."