With America at war and in need of a few good men, Jon Schoenherr expected a warm reception when he walked into an Army recruiting office in this Midwestern farm community, intending to enlist.

But a sergeant gave the 17-year-old some surprising news. "He told me I'd have to lose a little bit of weight," said Schoenherr, who dropped 50 pounds to qualify.

Besides terrorists, germ warfare and nuclear weapons, military officials increasingly worry about a different kind of threat -- troops too fat to fight.

Weight issues plague all branches of the military, from elite Marines to the Air Force, often lampooned as the "chair force" because of its many sedentary jobs.

Thousands of troops are struggling to lose weight, and thousands have been booted out of the service in recent years because they could not.

However, one of the biggest worries concerns those not even in uniform yet: Nearly two out of 10 men and four out of 10 women of recruiting age weigh too much to be eligible, a record number for that age group.

"This is quickly becoming a national security issue for us. The pool of recruits is becoming smaller," said Col. Gaston Bathalon, an Army nutrition expert. Unless weight rules are relaxed, "we're going to have a harder time fielding an Army."

Today's soldiers are supersized, averaging 37 pounds heavier than their Civil War counterparts. Military officials say that is not all bad, because most of it is muscle, not fat, and the result of better nutrition. "Large and in charge" makes soldiers look more formidable to the enemy, they note.

But at an obesity conference in Las Vegas last fall and in interviews since then, Bathalon and other military officials detailed the heavy burden that excess pounds are causing for some troops and taxpayers.

Weight problems add stress to already stressful jobs, costing many soldiers promotions and leading some to try desperate measures such as rubber suits and risky pills to shed pounds.

Problems do not end when active duty does, either. The Veterans Affairs health system increasingly is strained by vets piling on pounds and developing weight-related diseases such as diabetes.

Ironically, the big concern used to be soldiers not weighing enough. Congress passed the school lunch program after World War II, worried that too many high school students were malnourished and unfit to fight.

"This is the same deal in reverse. We've got young kids who are not going to be qualified for military service. They're either unfit or overfat," said Col. Karl Friedl, commander of the U.S. Army Research Institute of Environmental Medicine at Natick, Mass.

The institute has 170 doctors, dietitians, psychologists and other scientists who study military medical issues, from preventing heat exhaustion to coping with sleep deprivation. They view soldiers as specialized athletes whose physical condition can be a life-or-death matter. Increasingly, they deal with weight.

It starts with new recruits. Each branch of the service has its own entry rules, but according to federal weight guidelines, 43 percent of women and 18 percent of men in prime recruiting ages exceed screening weights for military service, Bathalon said.

Army standards are based on body fat, using a chart for body-mass index -- a ratio of weight and height -- as a screening tool. If soldiers or recruits exceed chart limits, body fat calculations are done using a formula based mostly on waist size.

Marines can be as much as 10 percent over weight standards to ship to boot camp.

"The Marines say, 'Send us anybody and we'll turn them into a Marine.' They're pretty successful at it," Friedl said.

Schoenherr, the Wisconsin Army recruit, was pretty successful, too. After weighing in at 215 pounds, he did his own boot camp during his senior year in high school, going to the recruiting center for 6 a.m. workouts, then downing a boiled egg or two and orange juice before heading to class.

Lunch would be "tuna fish right out of the can" or a low-carb wrap at school, he said. After school, he would lift weights. He is now 165 pounds and about to join a special forces unit.

"I've had some people who have lost close to 100 pounds to join," said Sgt. Chad Eske, his recruiter.

But often, making it into the military is just the start of the struggle. The military even has its own version of the "freshman 15" -- after basic training, Army women gain an average of 18 pounds in their first year and often have problems with annual weigh-ins that determine whether they can stay.

A survey Bathalon and others did of 1,435 troops referred to Fort Bragg Hospital for weight loss helps show the drastic measures some try. About three-fourths did things doctors recommend -- eating less, exercising more and downing more fruits and vegetables.

But many resorted to potentially harmful things. Nearly half tried using rubber suits or saunas to sweat off pounds, a third of men and half of women tried appetite suppressants, and one in five tried laxatives. Eleven percent of women and 6 percent of men had tried vomiting.

Half of the troops said stress was a reason they had gained weight, and half had come for help because they had been denied promotion.

"The Air Force is not escaping the national trends," Maj. Christine Hunter said at the Las Vegas obesity conference, showing a photograph of the new Baghdad Burger King, already the third busiest in the world.

About 1,500 troops were involuntarily separated, or kicked out, of the Air Force from 2000 to 2003 for failure to maintain weight, she reported.

In 2003 alone, more than 3,000 people were kicked out of all branches of the military for failing weight standards, Bathalon's study reports.

"You lose your income, you lose your retirement, you lose your medical benefits," he said.

Even those with long military careers sometimes develop weight problems afterward, burdening the VA health system, which treats about 5 million veterans each year, half of them older than 65.

They tend to be sicker than the general population. More than 70 percent are overweight and 33 percent are obese, said Richard Harvey, a health psychologist at the VA National Center for Health Promotion. Pain is the biggest reason they give for not exercising, and 31 percent say a disability prevents it, he said.

About 20 percent of veterans have diabetes, compared with 7 percent to 8 percent of the general population

"We knew that there were enormous costs with that," Harvey said, so he developed MOVE, a comprehensive program of psychological counseling, nutrition, exercise, medications and even sometimes surgery.

Parts of it started in October 2003 at 17 pilot sites, and the hope is to have a standardized program available to all veterans, said Steve Yevich, director of the VA health promotion center.

It will be a big improvement from 2001, when a survey by chief of staff Mary Burdick revealed that only 37 of the VA's 160 major medical centers had weight management programs, ranging from intense programs "down to just a little old dietitian sitting there," Yevich said.

Sending soldiers home healthy is the top goal, said Friedl of the Army research institute.

"It's not enough to recruit healthy young men and women and later return them safely to their families," he wrote in a recent medical journal article. "We now try to return them better than when they joined the Army with the promise that they will 'be all they can be.' "

Increasingly, that means weighing a little less.

Army Pfc. Jon Schoenherr, left, leads an exercise class outside the Army recruiting office in Watertown, Wis. Schoenherr lost 50 pounds in six months so he could qualify for boot camp.