This year marks the one hundredth anniversary of one of the United States Marine Corps’ (USMC)’s bloodiest battles of the Great War: the forgotten assault on Blanc Mont.

For years, the French Army had launched one forlorn attack after another on the fortress. The result: a bloodbath, thousands of men killed — gored on the seemingly impenetrable defenses of Blanc Mont.

Atop the high ground, the Germans had constructed multiple defensive belts. The maze of concrete bunkers, redoubts and machine-gun emplacements allowed the men to triangulate their shots, pouring enfilading fire on any assault force. In addition, a system of honeycombed trenches and tunnels connected the strongpoints, enabling the Germans to reinforce them or recapture overrun positions.

The French called in the shock troops of Pershing’s Army, the 2nd Division, which included the 4th Marine Brigade. Only days before the Marines’ attack, the Fourth French Army, which was led by the grizzled, one-armed General Henri Gouraud, had stalled in front of the ridge, the men exhausted. “General, this position is the key of all the German defenses of this sector, including the whole Rheims Massif,” Gouraud explained to Major General John Archer Lejeune, commander of the 2nd Division and future commandant of the Marine Corps. “If this ridge can be taken, the Germans will be obliged to retreat along the whole front thirty kilometers to the river Aisne. Do you think your division could effect its capture?”

Without hesitation, Lejeune informed Gouraud that the 2nd could seize the stronghold.

The 3rd Brigade (9th and 23rd Infantry Regiments) and 4th Brigade (5th and 6th Marines, as well as the 6th Machine Gun Battalion) were about to enter one of the most perilous kill zones on the Western Front. If they survived, the men would link up on the crest.

The story of that epic battle is retold in my new best-selling book, The Unknowns: The Untold Story of America’s Unknown Soldier and WWI’s Most Decorated Heroes Who Brought Him Home. Recently released, The Unknowns follows eight American heroes who accomplished extraordinary feats in some of the war’s most important battles. As a result of their bravery, these eight men were selected to serve as Body Bearers at the ceremony where the Unknown Soldier was laid to rest at Arlington Cemetery.

Several of the Body Bearers took part in the battle at Blanc Mount, as did Sergeant Edward Younger, the man who would later select the body that became America’s Unknown Soldier.

John Thomason, a member of the 49th Company of Marines, which The Unknowns follows throughout the war, described the moment when wave after wave of Marines and soldiers from the two brigades went over the top at 5:50 a.m. on October 3, charging into the teeth of German defenses: “For a moment, the sun shone through the murk, near the horizon—a smoldering red sun, banded like Saturn, and all the bayonets gleamed like blood. Then the cloud closed again.”

In front of them lay the result of the previous doomed French attack on the German trenches. Scores of dead soldiers formed “a wedge, thinning toward the point as they had been decimated, and at that point was a great bearded Frenchman, his body all a mass of bloody rags, who lay with his eyes fiercely open to the enemy and his outthrust bayonet almost in the emplacement.”

On the far right of the battlefield, Sergeant Edward Younger and his men went over the top and charged across the pock-marked no-man’s-land. Younger’s Company A led the way as German artillery thundered down around them, slaughtering several soldiers within minutes. Machine-gun fire pelted the men, pinning them down, until Private Frank J. Bart, one of the company runners, made a one-man charge on a machine-gun nest, grabbed the Chauchat inside and turned its deadly stream of bullets on the Germans.

Company A resumed its advance only to be checked again by a second nest. Undaunted, Bart repeated his previous feat, assaulting and destroying a second position. These valiant actions later earned the private a Medal of Honor.

Following in Bart’s wake, the rest of Company A, including Younger, slowly ascended Blanc Mont. Along the way, they sustained numerous casualties, some blown to bits by the German guns, their remains unaccounted for to this day. Company A’s wounded included Sergeant Younger, who went down with a Maxim machine-gun bullet to his thigh. The injury landed the Chicago native in a field hospital for weeks, but he fully recovered.

The evening of October 4, the 2nd’s Marines and soldiers linked up on the crest of Blanc Mont, an extraordinary feat of arms. French Field Marshal Philippe Pétain later said, “The taking of Blanc Mont is the single greatest achievement of the 1918 campaign.” Atop the ridge, the Americans continued to hack, blast and shoot the Germans fiercely fighting in concrete bunkers and trenches on the ridge.

But French ineptitude overshadowed the day’s glory. The allies failed to keep up with the 2nd, exposing its flanks. Taking advantage of the misstep, the Germans fired from all sides and brought in reinforcements in preparation for an all- out attempt to crush the Americans.

Furious, Lejeune cabled General John J. Pershing, commander of the American Expeditionary Forces (AEF), informing him he would rather resign than fight under French command if the Allies were not going to follow through on their commitments.

Atop Blanc Mont, the Marines and soldiers had now gone twenty-four hours without food or water. Desperately hungry and thirsty, the men scavenged the dead for their canteens while the Germans mercilessly counterattacked, shelled, and gassed them.

The 2nd was now within a mile of the German defenses at Saint-Étienne, but to get there, they would need to cross through open ground all while under constant fire from prepared positions.

As night fell, they seized the opportunity for a brief rest. Digging in on the northern side of Blanc Mont, the Americans girded for their coming attack the next day.

At dawn on October 4, the 5th Marines surged forward toward Saint-Étienne. As they stepped into the open ground, machine-gun bullets, artillery and gas tore into the sea of doughboys. Crumpled figures dropped to the ground. “The raw smell of blood was in the men’s nostrils,” Thomason remembered. “All hell broke loose” as German shrapnel rained down. “Singing balls and jagged bits of steel spattered on the hard ground like sheets of hail; the line writhed and staggered, steadied and went on, closing toward the center as the shells bit into it . . . red flashes, full of howling death.”

Thomason particularly remembered the fate of “a girlish, pink-cheeked lieutenant.” Naively enthusiastic to be charging into battle, the young officer had been gleefully swinging a brand new pair of field glasses. An instant later, an artillery shell flattened the lieutenant and the machine gunners beside him “into a mess of bloody rags, from which a bloody arm thrust upward, dangling the new field glasses.”

The Marines entered a deadly German kill zone which the Leathernecks dubbed “The Box.” Many would not return home alive. German gas shells dropped the advancing Marines, and enfilading fire from the left flank pelted the Leathernecks as the Germans tried to surround the Americans.

The situation was critical, and the battle was far from won. Only extraordinary heroism would enable the Americans to survive — and prevail — over the coming days.

Patrick K. O’Donnell is a bestselling, critically acclaimed military historian and an expert on elite units. He is the author of eleven books. The Unknowns is his newest, and it is featured in Barnes & Noble stores. O’Donnell served as a combat historian in a Marine rifle platoon during the Battle of Fallujah and speaks often on espionage, special operations, and counterinsurgency. He has provided historical consulting for DreamWorks’ award-winning miniseries Band of Brothers and for documentaries produced by the BBC, the History Channel, and Discovery. PatrickkODonnell.com @combathistorian