The Battle of Verdun, a massive German onslaught in northeastern France meant to gain a decisive final victory on the Western Front in World War I, began on this day in history, Feb. 21, 1916.

It is remembered by the French people today as a heroic patriotic defense of the homeland just 150 miles east of Paris. 

"On ne passe pas" are the words from Verdun etched in French national pride. 

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None shall pass. 

Scholars remember Verdun as the longest battle in modern military history — and for the unbelievable scope of the human carnage. 

Nearly 700,000 men were killed or wounded on both sides over 302 days of combat — a shocking average of about 2,300 casualties every day for nearly a year. 

Fought at the same time as the British-led Battle of the Somme north of Paris, it would ultimately create the stalemate on the Western Front that led to the entry of the United States into World War I in 1917. 

The goal of the attack on Verdun was to "bleed France white," proclaimed German army Chief of Staff Erich Von Falkenhayn.

It nearly succeeded. 

The battle began at 4 a.m. "with a massive artillery bombardment and a steady advance by troops the German Fifth Army under Crown Prince Wilhelm. Five days into battle, German forces captured Fort Douaumont, the largest and highest of the 19 forts protecting Verdun," writes the Imperial War Museum of London. 

"French military leaders declared Verdun could not be held if the east bank of the Meuse was lost and that French national morale would not survive the loss of the city."

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General Philippe Pétain was given command of French forces after the tragic losses in the first days of battle. He would mount a heroic defense. 

"Marshal Philippe Pétain, then a general and in command at Verdun, organized a system that was dubbed the ‘noria’ — or waterwheel — under which divisions from the whole of the French army were rotated through," the BBC wrote of the conflict in 2016. 

"It meant that vast numbers of French soldiers fought at different times at Verdun. Afterward, this was a crucial factor in concentrating the national memory."

The French press turned Verdun into "a sacred cause," the BBC added. 

"Any surrender was unthinkable." 

French convoys supplied the armies at the front over a single road under constant German bombardment. 

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The thoroughfare is still marked today as the Voie Sacree — the Sacred Way. 

The battle continues to haunt Verdun, over a century after the last soldier fell in battle.

One morbid cemetery sits on a hill beside a haunting memorial, the Douaumont Ossuary, with thousands of stark gray crosses — many with the words "un soldat inconnu" (an unknown soldier).

The bleakness of the structure reflects the sad "lost generation" zeitgeist of the World War I era.

It stands in sharp contrast to the pristine cemeteries of heroic white marble gravestones found at American military cemeteries in France and elsewhere around the world.

Shallow ditches, remnants of the trenches in which the soldiers lived and died in filth and misery, crisscross the woods and hills around the city.

Monuments to national war heroes and ordinary men who died in defense of Verdun line the roads in and around the city. 

The center of Verdun, stretching across both sides of a gentle stretch of the Meuse River, remains unusually somber even today. It’s as if the city is still in mourning. 

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The city center’s most notable landmark is the Citadel, a bunker carved into the hills where military leaders conducted the defense of Verdun. 

It's a subterranean fortress that served as the headquarters of the French effort. 

Visitors tour the citadel via a small rail car that rolls past recreated scenes of 1916: a damp, dark, ugly, cramped, chilly space packed with soldiers, frightened civilians, the dead and the dying. 

It reeked of death and human waste, according to contemporary accounts. But it remains a symbol of French survival and patriotism.

Marshal Pétain became a national hero for his determined leadership to protect the homeland. 

He would die a traitor. He led France's Nazi collaborationist government in Vichy during World War II. 

The Lion of Verdun was tried and convicted of treason after World War II. 

His death sentence was commuted, partly because of the legacy he forged in the defense of France 30 years earlier. 

He was "successively banal, then glorious, then deplorable, but never mediocre," General and Prime Minister Charles de Gaulle later said of Pétain.