MOUNT BUNDEY TRAINING AREA, Australia — First Lt. Marina A. Hierl watched a dozen Marines charge toward human silhouettes made of paper atop a nearby hill. Despite the early hour, the troops’ armored vests and camouflage uniforms were soaked with sweat. She stood back as they scrambled up the rocky incline, shouting and firing rifles.

“Push left,” she said after the squad completed its mock attack and assembled around her, gulping from canteens as they awaited feedback. “And make sure you’re communicating.”

It was a fairly routine instruction to Marines training for war, coming from a lieutenant in a role familiar to the men: a young, college-educated officer who had little experience but had direct oversight of their lives.

But Lieutenant Hierl is the first woman in the Marine Corps to lead an infantry platoon — a historic moment for a male-dominated organization that had fiercely opposed integrating female troops into combat, something that still unsettles many within the ranks.