The British expected little German resistance following the week-long bombardment. Instead, the Battle of the Somme became, as war poet Siegfried Sassoon described, a “sunlit picture of hell.” Of the 120,000 Allied troops—including those from Australia, India, South Africa, New Zealand, Newfoundland and Canada—who launched the initial attack, nearly 20,000 were killed, most of them in the first hour, and another 37,000 were wounded. Thirty-seven sets of British brothers lost their lives on the battle’s first day, and one man was killed every 4.4 seconds, making July 1, 1916, the bloodiest single day in the history of the British Army.

Following the horrific losses on the first day, the battle settled into a terrible war of attrition as the heat of summer gave way to autumn rains. “The conditions are almost unbelievable,” wrote Australian soldier Edward Lynch. “We live in a world of Somme mud. We sleep in it, work in it, fight in it, wade in it and many of us die in it. We see it, feel it, eat it and curse it, but we can’t escape it, not even by dying.” Allied forces launched no fewer than 90 attacks before British Commander-in-Chief Sir Douglas Haig called off the offensive on November 18. Over the course of the 141-day battle, the British advanced a total of only five miles.

British troops sustained 420,000 casualties—including 125,000 deaths—during the Battle of the Somme. The casualties also included 200,000 French troops and 500,000 German soldiers.

Throughout the autumn of 1916, more than 20 million Britons, nearly half the country’s population, flocked to cinemas to watch “The Battle of the Somme,” the first feature-length war documentary. Expecting a victory, the British War Office embraced the new medium of motion pictures and granted filmmakers Geoffrey Malins and John McDowell permission to record the battle in the hopes it would rally support for the war effort and aid recruiting. Incorporating both staged footage and real battle scenes captured between June 25 and July 9, the movie sparked controversy by depicting the brutality of war, including scenes of corpses being tossed into communal graves. “The Battle of the Somme” remains one of the most-watched films in British cinema history and blazed the trail for the movie industry’s ongoing obsession with war as a subject.

On September 15, the British deployed 32 Mark I tanks in an attack at Flers-Courcelette. Armed with either 6-pounder cannons or machine guns, the primitive tanks failed to break the military deadlock. Many of the armored vehicles, manned by eight-man crews, were grounded by mechanical breakdowns or ditched after failing to navigate the broken ground. The new instrument of war, which moved at 3 miles per hour, proved too slow to hold positions during counterattacks and was also susceptible to enemy grenades and armor-piercing rifles. As designs improved, the tank had a greater impact later in World War I.

In the belief that Britons would be more likely to volunteer to serve in World War I if they could serve alongside their friends, co-workers and neighbors, the British Army encouraged the formation of so-called “pals battalions,” which included groups ranging from London stock brokers to professional soccer players. The terrible losses sustained by these close-knit battalions at the Battle of the Somme, however, devastated the populations of entire communities. In the space of 30 minutes on the battle’s first day, 584 of the 720 members of the Accrington Pals were killed or wounded. The 600-man Grimsby Chums sustained more than 500 casualties on the battle’s opening day. As a result of the horrendous losses, the British Army gradually folded the “pals battalions” into other units.

Numerous British battalions were entering battle for the first time, and General Sir Henry Rawlinson issued an order that infantry troops were to advance at a walking pace in evenly spaced lines. Although many experienced officers ignored the order, thousands of the British who went over the tops of the trenches indeed walked steadily behind their officers, many of whom carried only revolvers or swagger sticks.

Captain Wilfred Nevill sought to encourage the four platoons of his 8th East Surrey Battalion to continue moving forward by presenting each with a soccer ball and promising a prize to whichever was first to kick it into the German trenches. One platoon painted “The Great European Cup” and “East Surreys v. Bavarians” on its soccer ball. When the whistles blew at “zero hour,” the cheering East Surreys kicked their balls as they moved forward, but they couldn’t escape the carnage. Seven officers were killed, and the 21-year-old Nevill was shot through the head in the first minutes of the battle. Two of the soccer balls were recovered from the battlefield near his body.

Grief from the horrific death toll filled tens of thousands of British homes—even 10 Downing Street, the residence of the prime minister. On September 15, 37-year-old Raymond Asquith—son of sitting British Prime Minister Herbert Asquith—was killed after being shot in the chest while leading an attack. (Two members of the British Parliament also lost their lives in the Battle of the Somme, and Harold Macmillan, who would serve as Britain’s prime minister from 1957 to 1963, was wounded twice while serving as an officer in the Grenadier Guards.)

In late September 1916, Bavarian Army corporal Adolf Hitler was dispatched with his unit to the Battle of the Somme, which he described as “more like hell than war.” Just days after Hitler’s deployment, a British shell exploded outside the entrance of the dugout near Bapaume, France, in which the dispatch runner was sleeping. While several of his fellow soldiers were killed, Hitler was wounded in the left thigh and, in spite of his protests, sent to convalesce in a German hospital before returning to his old regiment in early 1917.