The First World War began as a clash of 20th-century technology and 19th-century tactics, with the inevitably large ensuing casualties. By the end of 1917, however, the major armies, now numbering millions of men, had modernised and were making use of telephone, wireless communication, armoured cars, tanks, and aircraft. Infantry formations were reorganised, so that 100-man companies were no longer the main unit of manoeuvre; instead, squads of 10 or so men, under the command of a junior NCO, were favoured.
Artillery also underwent a revolution. In 1914, cannons were positioned in the front line and fired directly at their targets. By 1917, indirect fire with guns (as well as mortars and even machine guns) was commonplace, using new techniques for spotting and ranging, notably aircraft and the often overlooked field telephone. Counter-battery missions became commonplace, also, and sound detection was used to locate enemy batteries.
Germany was far ahead of the Allies in utilising heavy indirect fire. The German Army employed 150 and 210 mm howitzers in 1914, when typical French and British guns were only 75 and 105 mm. The British had a 6 inch (152 mm) howitzer, but it was so heavy it had to be hauled to the field in pieces and assembled. Germans also fielded Austrian 305 mm and 420 mm guns, and already by the beginning of the war had inventories of various calibers of Minenwerfer ideally suited for trench warfare.
Much of the combat involved trench warfare, in which hundreds often died for each yard gained. Many of the deadliest battles in history occurred during the First World War. Such battles include Ypres, the Marne, Cambrai, the Somme, Verdun, and Gallipoli. The Germans employed the Haber process of nitrogen fixation to provide their forces with a constant supply of gunpowder, despite the British naval blockade. Artillery was responsible for the largest number of casualties and consumed vast quantities of explosives. The large number of head wounds caused by exploding shells and fragmentation forced the combatant nations to develop the modern steel helmet, led by the French, who introduced the Adrian helmet in 1915. It was quickly followed by the Brodie helmet, worn by British Imperial and U.S. troops, and in 1916 by the distinctive German Stahlhelm, a design, with improvements, still in use today.
"Gas! Gas! Quick, boys!... Fitting the clumsy helmets just in time; But someone still was yelling out and stumbling, And flound'ring like a man in fire or lime... Dim, through the misty panes and thick green light, As under a green sea, I saw him drowning."- Wilfred Owen, DULCE ET DECORUM EST, 1917
The widespread use of chemical warfare was a distinguishing feature of the conflict. Gases used included chlorine, mustard gas and phosgene. Few war casualties were caused by gas, as effective countermeasures to gas attacks were quickly created, such as gas masks. The use of chemical warfare and small-scale strategic bombing were both outlawed by the 1907 Hague Conventions, and both proved to be of limited effectiveness, though they captured the public imagination.
The most powerful land-based weapons were railway guns weighing hundreds of tons apiece. These were nicknamed Big Berthas, even though the namesake was not a railway gun. Germany developed the Paris Gun, able to bombard Paris from over 100 kilometres (62 mi), though shells were relatively light at 94 kilograms (210 lb). While the Allies also had railway guns, German models severely out-ranged and out-classed them.