A short, scrawny 19-year-old Marine dashed through a deadly artillery barrage with a .45 in one hand and a grenade in the other. John Kelly, who served as a messenger on the front lines of the Western Front, came face-to-face with a German about to fire his machine gun.

With no time to think, Kelly raised his pistol and dispatched the enemy fighter. When one of the German’s comrades attempted to grab the fallen man’s weapon, Kelly tossed his grenade into the machine-gun nest. After the grenade detonated, the remaining eight Germans raised their hands in surrender.

Kelly took them all prisoner and marched the entire lot of POWs back through the artillery barrage into American lines.

For his actions, Kelly received not one, but two Medals of Honor, from the Army and the Navy. He also proved the truth of Marine Corps veteran John Ligato’s words: “The most ferocious fighting machine the world has ever seen is a 19-year-old pissed off Marine. Because you’ll take that kid from Detroit or Mississippi and you’ll train him the Marine Corps boot camp, and you’ll put him in a situation that’s foreign to him, and he will adapt and improvise and become that situation and deal with it.”

Kelly was fighting near “The Box,” a kill zone just over Blanc Mont Ridge in France. It was there that the USMC fought one of its deadliest battles one hundred years ago during the Great War.

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 Mont, as did Sergeant Edward Younger, the man who would later select the body that became America’s Unknown Soldier. Their mission: to assault a seemingly impregnable fortress that had stymied the French Army for years. Launching one forlorn attack after another, the French had lost thousands in a ceaseless series of bloodbaths.

Atop the high ground, the Germans had constructed multiple defensive belts. The maze of concrete bunkers, redoubts and machine-gun emplacements allowed the fighters 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. The one-armed French General Henri 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.

On October 3, 1918, the 3rd Brigade (9th and 23rd Infantry Regiments) and 4th Brigade (5th and 6th Marines, as well as the 6th Machine Gun Battalion) assaulted Blanc Mont. Five Marines, including Kelly, would receive the Medal of Honor for their heroics in the battle.

Kelly later saved a wounded Marine who had his leg blown off bringing him back to American lines. For his actions in combat, the teenaged private earned four Silver Stars, two Purple Hearts and numerous other medals.

In an extraordinary feat of arms, the 2nd Division took on the ridge. Kelly and the other Marines and Doughboys of the 2nd Division fought their way over the top and into The Box. There, flanked by the Germans on three sides, the men faced annihilation from German bullets, gas, and massive high explosive shells that could pulverize groups of men.

Naively enthusiastic to be charging into battle, a “girlish, pink-cheeked lieutenant” gleefully swung a brand new pair of field glasses. An instant later, an artillery shell flattened the young officer and the machine gunners beside him “into a mess of bloody rags, from which a bloody arm thrust upward, dangling the new field glasses.” Death in The Box came suddenly and swiftly.

“The raw smell of blood was in the men’s nostrils,” one Marine remembered. 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. Only extraordinary heroism would enable the Americans to survive — and prevail — over the coming days.

According to the official report, “The situation became so critical that it demanded immediate and bold action.” Marine Major George Hamilton, USMC, ordered his men of 1/5 who were in “The Box” to form up and push west, directly into the teeth of the oncoming assault. “The moment the battalion started forward, the machine guns opened up, and from then on their fire was incessant.”

With bullets whizzing past, the Leathernecks broke through the German counterattack and headed for the ridge outside Saint-Étienne that was their objective.

But the price was high. Hamilton’s force had dwindled to around 150 Marines from an original strength of about 1,000. Bodies lay in piles on the ground. The regimental history called October 4 “the bitterest single day of fighting the 5th Regiment experienced during the whole war.” Many of those who survived would struggle the rest of their lives with shell shock, now known as PTSD.

Exhausted, the Marines rested in shell holes about a mile from Saint-Étienne, steeling themselves for the coming assault.

On the evening of October 7–8, the 6th and 23rd Infantry Regiments, now preceded by a platoon of the 2nd Engineers that included Native American Body Bearer Corporal Thomas Saunders, went over the top once again to hit Saint-Étienne, the final line of German defenses at Blanc Mont. Braving fire, Saunders and his men employed wire cutters to clear a path through the thick web of German barbed wire.

The Americans eventually succeeded in taking the German strongpoint, and their efforts had an enormous impact on the entire front as one French Field Marshal later exclaimed, “The taking of Blanc Mont is the single greatest achievement of the 1918 campaign.” But when the battle was over, the 2nd Division had suffered thousands of casualties.

After the war, Private Kelly was honorably discharged from the Marine Corps. Like many veterans returning home, he found the transition difficult and drifted between jobs. In 1938, when asked by a reporter about his wartime exploits, the Marine responded, “The guys that are dead are greater heroes than I am. All I did was use a little headwork.”

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