QUANTICO, Va. -- It was 10 a.m., and the Marines trudging along theroad bordered by thick Virginia woods had been up for seven hoursalready.

Their uniforms were soaked with sweat, and their faces showed signs of the pain in their muscles. Their day was far from over.

Thedemanding training was a typical first day in the Marine Corps'Infantry Officer Course except for one thing: For the first time, twowomen were part of the class.

"The women are expected to doeverything that the men do," says Marine Col. Todd Desgrosseilliers, whocommands the organization responsible for basic Marine officer andinfantry training. "We haven't changed anything."

Women have beensteadily moving into many ranks previously barred to them, living atforward bases, flying combat aircraft and serving on submarine crews.Women remain barred from the infantry and other combat-arms specialties,but for the first time are being allowed to enter the Marines infantryofficer training.

Allowing the women to volunteer for the course is part of an"experiment" to determine how they perform in the rigorous regimen ofphysical and psychological stress that Marine infantry officercandidates are put through. Marine Corps' Infantry Officer Course is acourse in which about 25% of men don't make the cut or voluntarily dropout.

Critics say the move is taking gender equality too far. Theyworry that some efforts to accommodate women could lead to changingstandards and ultimately hurt military readiness.

"In the end,when all is said and done, what they should be focusing on is combateffectiveness," said Rep. Duncan Hunter, R.-Calif., a member of theArmed Services Committee. "Does it make us better at literally killingthe enemy? That's what their job is going to be."

The Marine Corpssay its experiment is an attempt to collect data for the Pentagon as itconsiders expanding the number of positions available to women in themilitary. The infantry is the most elemental and personal form ofwarfare, and remains off-limits to women.

For those who advocatethe breaking down of barriers throughout the military, the infantry isthe final frontier. Women who complete the Marine course will not becomeinfantry officers since Pentagon policy still prohibits it, but someare pushing for the ban to be lifted.

David Barno, a retiredthree-star Army general now a senior adviser at the Center for a NewAmerican Security, says the infantry is a brutal form of warfare and themilitary should consider any lifting of the ban carefully.

Infantrymenengage in close-in fighting, sometimes "with knives, rocks andshovels," Barno says. "I don't rule that out, but I think we should takea hard look at that."

Not just about brute strength

TheMarine Corps has rarely allowed journalists to view the InfantryOfficer Course. The Marines say assessing how the candidates deal withthe stress and uncertainty is crucial to selecting officers, so theydon't want them to know what to expect. USA TODAY agreed to withholddetails in return for access to the training.

The candidates,nearly all newly minted second lieutenants who have recently completedthe basic officer course, are dropped into the woods well before dawn.They must navigate through darkened woods using maps and compasses.

Carryingpacks and rifles, the prospects never stop moving throughout the day.They are given the briefest of instructions and are rebuffed if they askinstructors for further guidance. They don't even know the requirementsfor passing the course.

Physical endurance is only part of the course.

"We'renot just trying to see who is the most enduring or the toughest," Gen.James Amos, the Marine Corps commandant, told USA TODAY. "They have tobe able to make decisions under stress and duress."

For all theadvances in weaponry and tactics, the essence of the infantry haschanged little over the centuries. The infantry travels by foot, sleepsin the mud and engages in close combat. No amount of technology willchange that.

"The infantry is remarkably timeless," Desgrosseilliers says.

Infantryofficers carry an average of about 70 pounds of gear on their body incombat and can march for miles. That weight can nearly double that whenMarines are carrying crew-served weapons, such as mortars and heavymachine guns. They fight with what they carry on their backs.

MarineCapt. Brian Perkins kept a careful watch over a small group ofexhausted Marine lieutenants struggling through a series of pull-ups.

"She'sjust another student to me," Perkins said, referring to one of thewomen as she sweated through exercises. "The standard is the standard."

Menwho graduate from the Marine Infantry Officer Course will go on tocommand rifle platoons. Women who pass the course will go on to otherspecialties.

More than 280,000 women have deployed to Iraq andAfghanistan, but not in the infantry. Analysts point out that the natureof war has changed, blurring the distinction between "front line"troops like infantry and other support jobs.

"The performance ofwomen in combat is validated," says Desgrosseilliers, who was awarded asilver star, the military's third-highest award for heroism, inFallujah, Iraq. "They haven't been in the infantry though."

"Sometimeswe forget that even in Iraq and Afghanistan there have been manysituations where Marines are fighting with their bare hands against theenemy," said Maj. Scott Cuomo, director of the Infantry Officer Course."In one case, in a battle in Najaf, I was 50 feet from a Marineinfantryman killing the enemy with his knife."

Difference of opinions still remain

The U.S. military is committed to opening doors to more women.

Thispast spring the Pentagon made available more than 14,000 additionaljobs to women in the services. Women can now serve in staff positions insome combat-arms units and in units that serve along with combatorganizations, such as artillery or infantry.

About two-thirds ofpositions in the active duty Marine Corps were open to women, says thePentagon. However, women make up about 7% of the active duty MarineCorps and 13% of the Army.

Defense Secretary Leon Panetta hasordered the services to update him next month on how women performed inthe new jobs and on efforts to develop "gender-neutral physicalstandards" with an aim toward opening still more positions to women.

Developinggender-neutral standards raises the question of whether they would bemade less strenuous. Some advocates for putting women in the infantryhave suggested that the standards at the Marine officer training coursemay not be an accurate test of what it takes to be an infantryman.

NancyDuff Campbell, co-president of the National Women's Law center, saysthe Marines should first re-evaluate the standards before putting womenthrough the course.

"They're going at this backwards," Campbell says.

Not all women in the military say they should be allowed to serve in the infantry.

A female Marine officer with two combat tours had published an opinion piece in the Marine Corps Gazette saying the physical demands of infantry fighting were harmful to women physically.

"Iunderstand that everyone is affected differently; however, I amconfident that should the Marine Corps attempt to fully integrate womeninto the infantry, we as an institution are going to experience acolossal increase in crippling and career-ending medical conditions forfemales," Capt. Katie Petronio wrote.

She said women in theMarines were not clamoring for positions in the infantry, and that thedrive to have them serve is being orchestrated by a handful of groupsthat include the Pentagon's Defense Advisory Committee on Women in theService.

Women are held to different physical standards in recruittraining and other parts of the military, but doing so for the infantrywould be a mistake, infantry officers say.

The experiment willprovide the Marine Corps with information about how women perform ininfantry training. Amos says he wants data rather than "hunches."

'Deal with it'

Asthe afternoon drew on, Marines staggered along the roadway, some at aslow jog. Their faces beginning to take on what the Marines call "the1,000-yard stare." A woman hung from the pull-up bars, mustering thestrength to pull herself over the bar. Nearby, other Marines gruntedthrough exercises.

A steady rain fell, but infantrymen take a perverse delight in the tough conditions they face, says Cuomo.

"It's raining. The weather sucks. You're by yourself. You're hungry. Deal with it," he says.

Oneof the two women that started the 13-week course did not make it pastthe first day, which tests combat endurance. Neither did 27 of the 109men.

The Marine Corps did not release the names of the candidates.They did release a statement from the 24-year-old woman who passed: Shesaid she saw the training as an "incredible opportunity" for women.

"It'sabout the balance between mental and physical toughness," Perkins saidof those who have what it takes to be infantry officers. "You can see itin their eyes."