The First World War was the first great war fought with the full power of industrial economies. Factories that had made sewing machines and cars turned out shells, rifles and engines by the million. Most battlefield deaths and wounds were caused by artillery, not by bullets or bayonets: by common estimates, shellfire caused well over half of all wounds. A soldier's deadliest enemy was often a gun he never saw, firing from several kilometres away.
Machine guns and artillery
The machine gun, developed in the 1880s, could fire hundreds of rounds a minute. A few gunners in a protected position could stop a battalion advancing across open ground. European armies had used such weapons against poorly armed opponents in colonial wars, but in 1914 they turned them on each other, with devastating results.
Artillery grew more powerful and more numerous every year. Before a major attack, guns might fire for days. Over time gunners learned better methods. The creeping barrage was a curtain of shellfire that moved forward just ahead of advancing infantry, keeping defenders' heads down until the attackers reached them. By 1917 and 1918, gunners could locate enemy guns by their sound and flash and could hit targets accurately without warning shots to find the range, so surprise became possible again.
Poison gas
The Hague Conventions of 1899 and 1907 had banned projectiles meant to spread poisonous gases. On 22 April 1915, near Ypres in Belgium, the German army released chlorine gas from cylinders, and a greenish cloud drifted over French and Algerian troops, who fled or choked. The chemist Fritz Haber directed the program. The Allies soon replied with gas of their own. Later came phosgene, which was harder to detect, and mustard gas, first used in 1917, which burned the skin, eyes and lungs and lingered in the soil for days.
Gas masks improved quickly, and gas killed a fairly small share of the war's dead, perhaps around 90,000 out of roughly 1.3 million gas casualties. But it caused terror out of proportion to its death toll, and many survivors lived with damaged lungs for the rest of their lives. The poet Wilfred Owen, an officer who had seen a gas attack, described a man dying after failing to get his mask on in time:
If you could hear, at every jolt, the blood / Come gargling from the froth-corrupted lungs ... / My friend, you would not tell with such high zest / To children ardent for some desperate glory, / The old Lie: Dulce et decorum est / Pro patria mori.
The Latin, from the Roman poet Horace, means "It is sweet and fitting to die for one's country." Owen's poem "Dulce et Decorum Est" attacked the idea that dying in war is glorious. Owen was killed on 4 November 1918, a week before the war ended, and most of his poems were published after his death.
Tanks, aircraft and submarines
The British developed an armoured vehicle on caterpillar tracks to cross trenches and crush wire, code named the "tank" to pass it off as a water carrier. Tanks first went into battle on the Somme in September 1916, at Flers-Courcelette. Early models broke down often, but massed tanks at Cambrai in 1917 showed their promise.
Aircraft began the war as scouts. Soon they spotted targets for the artillery, photographed enemy trenches, fought each other in the air and dropped bombs. German Zeppelin airships and later Gotha bombers raided Britain, killing about 1,400 civilians in all and spreading the idea that cities far from the front were targets too.
At sea, Britain's navy blockaded Germany, and Germany struck back with submarines, called U-boats. In May 1915 a U-boat sank the British liner Lusitania, killing nearly 1,200 people, including 128 Americans. American outrage pushed Germany to restrict its submarine attacks for a time. The only great clash of battle fleets, at Jutland in 1916, ended without a decisive result, though Germany's surface fleet rarely ventured out again.
No single weapon ended the deadlock. What finally broke the stalemate in 1918 was combined arms: infantry, artillery, tanks and aircraft working together, with infantry moving in small groups armed with light machine guns and grenades.
Worked example: why new weapons did not end the stalemate
Suppose you command an attack in 1915 and someone offers you a war winning weapon: gas. On the first day at Ypres it opened a gap several kilometres wide. Yet the Germans had brought too few reserves to exploit it, because they saw it as an experiment, and within days Allied troops were improvising masks from cloth soaked in water or urine. Within months both sides had gas and both had masks, and the advantage was gone.
The same pattern repeated with tanks and aircraft. A new weapon gave a brief edge, the enemy adapted, and the stalemate returned. What broke it in 1918 was not one invention but organization: commanders who had learned from years of failure, combining every arm under a single plan. The lesson reaches beyond war. In any contest between adapting opponents, a single clever tool rarely decides the outcome, while learning and coordination often do.