At 7:12 a.m. on the morning of February 21, 1916, a shot from a German Krupp 38-centimeter long-barreled gun—one of over 1,200 such weapons set to bombard French forces along a 20-kilometer front stretching across the Meuse River—strikes a cathedral in Verdun, France, beginning the Battle of Verdun, which would stretch on for 10 months and become the longest conflict of World War I.

By the beginning of 1916, the war in France, from the Swiss border to the English Channel, had settled into the long slog of trench warfare. Despite the hard conditions in the trenches, Erich von Falkenhayn, chief of staff of the German army, believed that the key to winning the war lay not in confronting Russia in the east but in defeating the French in a major battle on the Western Front.

In December 1915, Falkenhayn convinced the kaiser, over the objections of other military leaders such as Paul von Hindenburg, that in combination with unrestricted submarine warfare at sea, a major French loss in battle would push the British—whom Falkenhayn saw as the most potent of the Allies—out of the war.

The chosen mark of Falkenhayn’s offensive was the fortress city of Verdun, on the Meuse River in France. The city was selected because in addition to its symbolic importance—it was the last stronghold to fall in 1870 during the Franco-Prussian War—it was possible to attack the fortress city from three sides, which made it a good strategic target.

Ignoring intelligence that warned of a possible German attack in the region, French command had begun in 1915 to strip its forces at Verdun of the heavy artillery essential to defensive warfare, choosing instead to focus on an offensive strategy masterminded by General Ferdinand Foch, the director of the army’s prestigious War College, and dubbed Plan XVII. Thus the German attack of February 21 caught the French relatively unprepared.

From the beginning, the Battle of Verdun resulted in heavy losses on both sides. Falkenhayn famously admitted that he did not aim to take the city quickly and decisively, but to bleed the French white, even if it meant an increased number of German casualties. Within four days of the start of the bombardment on the Meuse, the French forward divisions had suffered over 60 percent casualties; German losses were almost as heavy.

After a few quick German gains of territory, the battle settled into a stalemate, as casualties swiftly mounted on both sides. The newly promoted French commander in the region, Henri-Philippe Petain, was determined to inflict the maximum amount of damage on the German forces, famously pledging to his commander-in-chief, Joseph Joffre, that, They shall not pass.

By the latter half of 1917, German resources were stretched thinner by having to confront both a British-led offensive on the Somme River and Russia’s Brusilov Offensive on the Eastern Front. In July, the kaiser, frustrated by the state of things at Verdun, removed Falkenhayn and sent him to command the 9th Army in Transylvania; Paul von Hindenburg took his place. Petain had been replaced in April by Robert Nivelle, who by early December had managed to lead his forces in the recapture of much of their lost territory.

From December 15 to 18, the French took 11,000 German prisoners; on December 18, Hindenburg finally called a stop to the German attacks after ten long months. With a German death toll of 143,000 (out of 337,000 total casualties) and a French one of 162,440 (out of 377,231), Verdun would come to signify, more than any other battle, the grinding, bloody nature of warfare on the Western Front during World War I.