and . 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. The Germans also fielded Austrian and guns and, even at the beginning of the war, had inventories of various calibres of Minenwerfer, which were ideally suited for trench warfare. On 27 June 1917 the Germans used the biggest gun in the world, Batterie Pommern, nicknamed "Lange Max". This gun from Krupp was able to shoot 750 kg shells from Koekelare to Dunkirk, a distance of about . Much of the combat involved trench warfare, in which hundreds often died for each metre gained. Many of the deadliest battles in history occurred during World WarI. 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 US troops, and in 1916 by the distinctive German Stahlhelm, a design, with improvements, still in use today. The widespread use of chemical warfare was a distinguishing feature of the conflict. Gases used included chlorine, mustard gas and phosgene. Relatively 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 (as opposed to tactical bombing) were both outlawed by the Hague Conventions of 1899 and 1907, and both proved to be of limited effectiveness, though they captured the public imagination. The most powerful land-based weapons were railway guns, weighing dozens of tons apiece. The German version were nicknamed Big Berthas, even though the namesake was not a railway gun. Germany developed the Paris Gun, able to bombard Paris from over , though shells were relatively light at 94 kilograms (210 lb). Trenches, machine guns, air reconnaissance, barbed wire, and modern artillery with fragmentation shells helped bring the battle lines of World WarI to a stalemate. The British and the French sought a solution