The Battle of the Somme was one of the largest battles of World War I, and among the bloodiest in all of human history. A combination of a compact battlefield, destructive modern weaponry and several failures by British military leaders led to the unprecedented slaughter of wave after wave of young men.

“The German doctrine was that not a single yard of ground should be surrendered and the French and the British were determined to never stop the attack,” says Spencer Jones, senior lecturer in Armed Forces and War Studies at the University of Wolverhampton.

“So both sides were locked into a frighteningly small area onto which an enormous amount of firepower was poured.”

On the first day alone, the British endured more than 57,000 casualties. The nearly 20,000 British troops killed on day one of the infantry assault was so high it remains the single worst day in British military history. By the 141-day battle’s end, the Allies and Central Powers suffered more than a million casualties combined.

The campaign, staged along an 18-mile stretch around the Somme River in France, was a joint French and British offensive to expel German forces. The location and timing of the attack was also intended to relieve pressure on Verdun, where French troops were enduring a punishing German attack.

British General Sir Douglas Haig ordered a week-long artillery bombardment of more than a million shells starting on June 24. The hail of shells was intended to wipe out German barbed wire, front-line trenches, artillery—and the morale of the German army.

“We were informed by all officers from the colonel downwards that after our tremendous artillery bombardment there would be very few Germans left to show fight,” recalled Lance Cpl. Sidney Appleyard of Queen Victoria’s Rifles.

In fact, more than half of the artillery shells failed to detonate, most of the German dugouts remained undamaged and their wire barricades remained largely intact.

The terrain around the Somme was made up primarily of chalk, which the Germans had found well-suited for trench warfare, including a deep network of fortified trenches, complete with rear supply fortifications and buried communication lines.

As British and French infantry went “over the top” of their trenches starting at 7:30 a.m., expecting little German resistance, they were mowed down by German artillery and machine gun fire. The advancing soldiers could not move quickly since most were carrying some 60 pounds of gear, including picks, sandbags and shovels to shore up enemy positions that they believed had been blown out by artillery and abandoned.

Also exacerbating the slaughter was the fact that the advancing troops, in seeking out openings in the German barbed wire, ended up clustering at the gaps, making them easy targets.

One German machine gunner recalled, “When we started to fire we just had to load and reload…They went down in their hundreds. We didn’t have to aim, we just fired into them.”

Despite the shockingly heavy losses of July 1, General Haig and other military leaders resumed the attacks the next day—and the next. As Jones said, “There was a remarkable refusal to give up. That led to the battle’s overall horrifying death toll.”

Over the course of the campaign, both sides fired artillery shells by the tons, unleashed streams of machine gun fire, sprayed chemical weapons, fired flamethrowers, and British troops deployed tanks for the first time.

Casualties just kept rising as the Somme became a grueling battle of attrition. As Jones said, “Human flesh is powerless to withstand that amount of destruction.”

British commanders swiftly learned from their devastating showing in the early days of the battle and adjusted their tactics. In the end, the Allied forces advanced a mere six miles. But the devastating losses on both sides showed that any territory fought on the Western Front would be hard-won.

In the wake of the Somme's and other battles’ grim death tolls, Germany eventually shifted its strategy away from the Western Front to initiate submarine warfare, which played a part in bringing the United States into the war.

The Battle of the Somme, says Gary Sheffield, Professor of War Studies at the University of Wolverhampton, “was a ghastly human experience,” but, he says, “it was not futile and it provided a stepping stone to victory in 1918.”