Before dawn on the morning of September 15, 1916, a deep rumble suddenly shook the French countryside as the British launched a major offensive to capture the village of Courcelette. Waves of British soldiers vaulted over the top of their trenches accompanied by something that no German soldier—nor any soldier in battle, for that matter—had ever seen. For rising out of the mud and the darkness came a fleet of strange mechanical beasts creeping forward on conveyor-belt-like tracks wrapped around their wheels.

The British military hoped that its new weapon—the tank—could finally break the deadly impasse of the Battle of the Somme. Early in the war, British Army Colonel Ernest Swinton proposed the development of an armored vehicle that could traverse difficult terrain, and First Lord of the Admiralty Winston Churchill approved the development of the so-called “landships” in early 1915. Much of the design work was done by two men working secretly inside a Lincoln, England, hotel room near a threshing machine manufacturer commissioned to build the prototypes.

The first prototype, “Little Willie,” was tested in September 1915 to poor results. A second prototype, “Big Willie,” achieved much greater success and was deemed ready for battle. Swinton formed the Heavy Section of the Machine Gun Corps in March 1916 to train 500 recruits to operate the vehicles. Production workers noted that the oddly-shaped shells of the vehicles resembled water tanks, and they were secretly shipped to the front lines in crates labeled “tanks.” The name stuck.

Desperate to end the stalemate of the Battle of the Somme, the British rushed the new weapon into battle. The tanks lacked sufficient testing and their crews ample training. Tank crews had never trained with infantry units for this new type of warfare, and some had never even fired practice rounds from their guns. Of the 49 Mark I tanks sent to the battlefield, 17 were sidelined by mechanical malfunctions even before the offensive on Courcelette could begin.

While the 32 tanks sent into battle mowed down barbed wire, many struggled to cross the trenches and artillery craters in no-man’s land. Some were forced to be ditched in the broken ground. The new instrument of war, which moved at a walking pace of fewer than 4 miles per hour, proved too slow to hold positions during counterattacks.

The eight-man crews, including two drivers, who squeezed inside the tanks sweated through insufferable heat and were forced to communicate with hand signals thanks to the engine’s deafening din. They were alarmed to discover that the skin of the 29-ton machine also offered less protection than they had hoped. While some German troops, not knowing what to do against these unfamiliar terrestrial ironclads, ran away, others unleashed machine gun and pistol fire, grenades and artillery at the tanks. A barrage of bullets pierced the tanks’ armor, and scalding metal shards sprayed the crews like shrapnel, burning their hands and faces.

“We steamed ahead, squashing dead Germans as we went,” reported tank commander Lieutenant Basil Henriques of his vehicle’s progress. “As we approached the German line they let fire at us with might and main. At first no damage was done and we retaliated, killing about 20. Then a smash against my flap at the front caused splinters to come in and the blood to pour down my face. Another minute and my driver got the same.”

Armed with either 6-pounder cannons or machine guns, the primitive tanks failed to break the military deadlock of the Battle of the Somme. Only 9 tanks reached enemy territory, and only 3 returned to British lines, all too badly damaged to ever see action again. Nevertheless, British military leaders saw the potential of the new war machines. British Commander-in-Chief Sir Douglas Haig ordered the production of hundreds more.

“The tanks had limited success on that first day in military terms, however their success in terms of psychology shouldn’t be underestimated,” David Willey, curator of The Tank Museum in Bovington, England, told the BBC. “The German troops were terrified of these machines and for the British, the tanks were a huge morale boost. This was a British invention, designed to save soldiers’ lives, and it gave people hope, both on the front line and back at home.”

The Mark I was remodeled several times before war’s end, and better-designed British tanks proved decisive at the Battle of Cambrai in November 1917. Before the end of World War I, German tanks—some of them restored versions of captured British tanks—also took to the battlefield. The tank would come to dominate 20th-century warfare, particularly during World War II when Nazi Panzer divisions used them to devastating effect on Blitzkrieg attacks across Europe.