At 7:30 a.m., the British launch a massive offensive against German forces in the Somme River region of France. During the preceding week, 250,000 Allied shells had pounded German positions near the Somme, and 100,000 British soldiers poured out of their trenches and into no-man’s-land on July 1, expecting to find the way cleared for them. However, scores of heavy German machine guns had survived the artillery onslaught, and the infantry were massacred. By the end of the day, 20,000 British soldiers were dead and 40,000 wounded. It was the single heaviest day of casualties in British military history. The disastrous Battle of the Somme stretched on for more than four months, with the Allies advancing a total of just five miles.

When World War I broke out in August 1914, great throngs of British men lined up to enlist in the war effort. At the time, it was generally thought that the war would be over within six months. However, by the end of 1914 well over a million soldiers of various nationalities had been killed on the battlefields of Europe, and a final victory was not in sight for either the Allies or the Central Powers. On the Western Front—the battle line that stretched across northern France and Belgium—the combatants had settled down in the trenches for a terrible war of attrition. Maimed and shell-shocked troops returning to Britain with tales of machine guns, artillery barrages, and poison gas seriously dampened the enthusiasm of potential new volunteers.

READ MORE: Why Was the Battle of the Somme So Deadly?

With the aim of raising enough men to launch a decisive offensive against Germany, Britain replaced voluntary service with conscription in January 1916, when it passed an act calling for the enlistment of all unmarried men between the ages of 18 and 41. After Germany launched a massive offensive of its own against Verdun in February, Britain expanded the Military Service Act, calling for the conscription of all men, married and unmarried, between the ages of 18 and 41. Near the end of June, with the Battle of Verdun still raging, Britain prepared for its major offensive along a 21-mile stretch of the Western Front north of the Somme River.

For a week, the British bombarded the German trenches as a prelude to the attack. British Field Marshal Douglas Haig, commander of the British Expeditionary Force, thought the artillery would decimate the German defenses and allow a British breakthrough; in fact, it served primarily to remove the element of surprise. When the bombardment died down on the morning of July 1, the German machine crews emerged from their fortified trenches and set up their weapons. At 7:30 a.m., 11 British divisions attacked at once, and the majority of them were gunned down. The soldiers optimistically carried heavy supplies for a long march, but few made it more than a couple of hundred yards. Five French divisions that attacked south of the Somme at the same time fared a little better, but without British success little could be done to exploit their gains.

After the initial disaster, Haig resigned himself to smaller but equally ineffectual advances, and more than 1,000 Allied lives were extinguished for every 100 yards gained on the Germans. Even Britain’s September 15 introduction of tanks into warfare for the first time in history failed to break the deadlock in the Battle of the Somme. In October, heavy rains turned the battlefield into a sea of mud, and on November 18 Haig called off the Somme offensive after more than four months of mass slaughter.

READ MORE: Life in the Trenches of World War I

Except for its effect of diverting German troops from the Battle of Verdun, the offensive was a miserable disaster. It amounted to a total gain of just 125 square miles for the Allies, with more than 600,000 British and French soldiers killed, wounded, or missing in the action. German casualties were more than 650,000. Although Haig was severely criticized for the costly battle, his willingness to commit massive amounts of men and resources to the stalemate along the Western Front did eventually contribute to the collapse of an exhausted Germany in 1918.