Under intense pressure to trim its budget, the Army is dismissing a rising number of soldiers who do not meet its fitness standards, drawing from a growing pool of troops grappling with obesity.

Obesity is now the leading cause of ineligibility for people who want to join the Army, according to military officials, who see expanding waistlines in the warrior corps as a national security concern.

Between 1998 and 2010, the number of active-duty military personnel deemed overweight or obese more than tripled. In 2010, 86,186 troops, or 5.3 percent of the force, received at least one clinical diagnosis as overweight or obese, according to the Armed Forces Health Surveillance Center.

The trend has prompted the military to reexamine its training programs and is driving commanders to weed out soldiers deemed unfit to fight. “A healthy and fit force is essential to national security,” said Cmdr. Leslie Hull-Ryde, a Pentagon spokeswoman. “Our service members must be physically prepared to deploy on a moment’s notice anywhere on the globe to extremely austere and demanding conditions.”

During the first 10 months of this year, the Army kicked out 1,625 soldiers for being out of shape, about 15 times the number discharged for that reason in 2007, the peak of wartime deployment cycles.

Growing numbers of the military are failing fitness standards.

Under a mandate to reduce the force by tens of thousands in coming years, the Army has instructed commanders to make few exceptions when it comes to fitness, a strategy it also employed during the period after the 1991 Persian Gulf War.

“During a war period, when we were ramping up, the physical standards didn’t have a lot of teeth because we needed bodies to go overseas, to fill platoons and brigades,” said Stew Smith, a former Navy SEAL and fitness expert who has designed workout routines for service members and law enforcement personnel struggling to meet workplace fitness standards. “During a period of drawdown, everything starts getting teeth, and that’s kind of where we are again.”

The Army dismissed thousands of soldiers for being overweight after Desert Storm ended in 1991. The following year, it discharged more than 3,000, the highest number removed on those grounds since 1984. The practice dropped dramatically as the Iraq war raged. In 2007, the most violent year of the war, 112 soldiers were let go for being overweight.

Retired Lt. Gen. Mark Hertling said he was floored by what he found in 2009 when he was assigned to overhaul the Army’s training system. Seventy-five percent of civilians who wanted to join the force were ineligible, he said. Obesity was the leading cause.

“Of the 25 percent that could join, what we found was 65 percent could not pass the [physical training] test on the first day,” he said in a recent speech. “Young people joining our service could not run, jump, tumble or roll — the kind of things you would expect soldiers to do if you’re in combat.”

As the Iraq and Afghanistan wars strained the military, the Army granted waivers to recruits who would normally not be eligible — for example, people who were overweight or who had criminal records. But now, under orders to reduce the active-duty force from 570,000 to 490,000 by 2017, the Army has ordered commanders to weed out substandard troops. “We will use the drawdown as an opportunity to shape our Army by ensuring that we retain only the very best soldiers,” Army Secretary John M. McHugh wrote in a Feb. 2 memo on retention initiatives.

The Navy, Marines and Air Force did not provide data on dismissals for failing to meet fitness standards. Those services, which are far smaller than the Army, are not being forced to shrink as quickly and drastically as the Army.

The strict enforcement of fitness requirements in the Army has cast a spotlight on its fitness test, which some soldiers say unfairly labels strong, capable soldiers as unfit. The two-pronged test involves a physical endurance portion during which troops must do sit-ups, push-ups and a brief run. The second phase is a height and weight measurement. The criteria for both vary depending on age.

Some soldiers who are muscular are astonished to fail the height-weight standard. The first time he took the test, Staff Sgt. Ammiel Banayat was surprised to find that he was over the limit. He is 5 feet 5 inches tall and weighs just more than 160 pounds. To override the standard, he was subjected to a body-fat index test that takes into account tape measurements of the neck and waistlines.

“The first time it happened, I was petrified,” the Arizona National Guardsman said. “Even though I still passed the test, just the fact that I didn’t pass the height and weight test was terrifying.”

Smith, the former Navy SEAL, said the bulk of people struggling with weight issues are simply the product of a generation that has become increasingly sedentary and accustomed to large food portions. Some of the soldiers fighting for their jobs, however, have gained weight at least in part as a result of injuries, he said.

On blogs and forums, military personnel and spouses have been critical of the dismissals, arguing that soldiers who put their lives on the line in combat are being treated as expendable.

A writer using the screen name Army Mom complained on a court-martial defense lawyer’s blog that her son was being kicked out for being overweight despite having suffered a knee injury in Iraq. “My son fought for this country and has a wife and 3 young children, the youngest a month old and they are now homeless,” she wrote in September.

A soldier, writing anonymously in the comments section of the blog, sought advice, saying he was being dismissed for putting on weight after surgery. A second soldier posted a comment saying he gained weight after 19 years in the service when he started taking medicine for depression.

“The Army doctor swears that the medicine does not make [me] gain weight and it is the depression that made me get careless and gain the weight,” the soldier wrote. “The company commander wants to separate me and really there is nothing I can do — just to lose the weight.”