The medics helped Sgt. Janiece Marquez into a chair and started to treat her sprained ankle. Marquez, 25, had tripped over a rock on one of the dark paths in the camp. She had just run two miles during the physical fitness test and marched at least six miles carrying a 35-pound rucksack that evening. Now she could barely walk.

One of the medics looked at her ankle.

“Are you going to be able to ruck tomorrow?”

“Absolutely,” Marquez said.

“What if I tell you the next day you’re going to go about 25 miles? Are you ready for that? Do you think you can physically do it?”

What Marquez knew for certain was that she wasn’t going to quit. And that refusal to give up was what the evaluators, all special operations soldiers, were looking for in the 55 selectees here at Camp Mackall, a former World War II training base near Fort Bragg tucked into the pine forests of central North Carolina. They were being considered for elite, all-female teams trained to build relationships with Afghan women.

The evaluators wanted the Army’s best female soldiers. The toughest — mentally and physically — and the sharpest intellectually. The next 100 hours would not only test the soldiers’ ability to run and march, but also how well they thought on their feet and adapted to the unknown.

With a throbbing ankle and many more back-breaking marches with heavy rucksacks and lung-burning runs ahead of her, Marquez got up and limped across camp.

***

While Congress still bans women from serving in combat units, the soldiers selected from this group will serve alongside the Army’s most elite units on the battlefield. The Army has never selected women to do a mission because of their sex, until now.

It is recruiting female soldiers to work closely with Special Forces teams and Ranger units during raids. Because women and children are often held in a separate room while soldiers search the compound, these teams go into villages in Afghanistan to buil...