The struggle to entice Army soldiers and Marines to stay in the military, after four years of war in Iraq, has ballooned into a $1 billion campaign, with bonuses soaring nearly sixfold since 2003.

The size and number of bonuses have grown as officials scrambled to meet the steady demand for troops on the battlefields in Iraq and Afghanistan and reverse sporadic shortfalls in the number of National Guard and Reserve soldiers willing to sign on for multiple tours.

Besides underscoring the extraordinary steps the Pentagon must take to maintain fighting forces, the rise in costs for re-enlistment incentives is putting strains on the defense budget, already strapped by the massive costs of waging war and equipping and caring for a modern military.

The bonuses can range from a few thousand dollars to as much as $150,000 for very senior special forces soldiers who re-enlist for six years. All told, the Army and Marines spent $1.03 billion for re-enlistment payments last year, compared with $174 million in 2003, the year the war in Iraq began.

The Associated Press compiled and analyzed the budget figures from the military services for this story.

"War is expensive," said Col. Mike Jones, who oversees retention issues for the National Guard. "Winning a war, however, is less expensive than losing one."

The soaring budget for re-enlistment bonuses — particularly for the Guard and Reserves, which have seen the most dramatic cost increases — has prompted some observers to question whether the country can still afford its volunteer force.

"I believe the whole issue of the affordability of the volunteer force is something we need to look at," said Arnold Punaro, who heads an independent panel established by Congress to study the National Guard and Reserves.

The higher bonuses come as support for the war continues to wane both in Congress and with the American public. That decline is fueling concerns that more soldiers will leave the military under pressure from families who fear the rising death toll and are weary of the lengthy and repeated overseas deployments. The Iraq war has claimed the lives of at least 3,280 U.S. troops to date.

Incentives for Army Guard and Reserve members combined have skyrocketed from about $27 million in 2003 to more than $335 million in 2006.

The active Army, meanwhile, poured more than $600 million into these payments last year, a six-fold increase from $98 million in 2003. The Army gave two out of every three soldiers who re-enlisted a bonus last year, compared to less than two in 10 who received one during 2003.

Those who don't get bonuses are generally in jobs that are not in high demand or are not in war zones. For example, certain artillery crewmembers who re-enlisted outside Afghanistan or Iraq would receive no bonus, said Army spokesman Lt. Col. Bryan Hilferty.

Bonuses for Marines have nearly doubled, from about $50 million in 2003 to nearly $90 million in 2006.

The incentives help the military compete with private employers who often pay much higher salaries, Hilferty said.

"Soldiers with valuable skills and experience are aggressively sought after by industry," Hilferty said. He said while the extra money is important, "people don't re-enlist in a wartime Army for $13,000. ... If soldiers didn't think they were doing the right thing for the right reason, they would get out and get a job back home."

He said soldiers with special skills can get bonuses between $10,000 and $30,000, with a select few eligible for payments up to $50,000. Only very few highly qualified special forces soldiers would get the top bonus of $150,000. Nearly all soldiers deployed to Iraq, Afghanistan and Kuwait get a maximum of $15,000 for re-enlisting, just a bit more than the average.

Bonuses for Marines in certain critical specialties can go as high as $60,000 for a new four-year tour. On average a Marine who re-enlists this year can receive as much as $24,000. About eight in 10 Marines with up to six years of service will get a bonus this year, as will more than half of those with six to 14 years in the Corps.

Punaro, chairman of the Commission on the National Guard and Reserves, calls the soaring costs "a ticking time bomb."

"My instinct tells me ... that the Guard and Reserve will continue to be a real bargain for the taxpayer" because the costs for the active duty military have gone up a lot more, he said.

So far, the extra cash appears to be working. The active Army, the Guard and the Army Reserve are all on track to meet their re-enlistment goals for the fiscal year that will end Sept. 30.

Sgt. 1st Class Richard Doran, who works full-time for the Guard, signed on for another six-year tour late last year, just before he returned home from Iraq. That not only gives him the $15,000 bonus but also makes it tax-free because he was on the battlefront when he re-enlisted.

"It helps a lot of guys out," said Doran. "And I think it does sway some of the decisions to stay in when guys are on the fence trying to decide."

But for some who have been sent to war as many as three times, the money isn't enough.

"We had some that, once we got back, opted to say goodbye and just leave. Some guys said the money did play a part in their decision to stay, others said the $15,000 wasn't worth it."

Jones of the Guard said boosting the maximum re-enlistment bonus from $5,000 to $15,000 caused most of the budget increase. And, he said, more soldiers signed up than anticipated.

"When we're at peace, and when we're not deploying units, the bonuses probably don't need to be what they are today," said Jones. "When the risks are lowered, the reward would be lowered. But one of reasons we struggled in 2005 and 2004 is because we were slow as a nation to increase the rewards at the same time as we increased the risk."