<P> All sides had signed the Hague Conventions of 1899 and 1907, which prohibited the use of chemical weapons in warfare . In 1914, there had been small - scale attempts by both the French and Germans to use various tear gases, which were not strictly prohibited by the early treaties but which were also ineffective . The first use of more lethal chemical weapons was against the French near the Belgian town of Ypres . </P> <P> Despite the German plans to maintain the stalemate with the French and British, Albrecht, Duke of Württemberg, commander of the 4th Army planned an offensive at Ypres, site of the First Battle of Ypres in November 1914 . The Second Battle of Ypres, April 1915, was intended to divert attention from offensives in the Eastern Front and disrupt Franco - British planning . After a two - day bombardment, the Germans released a cloud of 168 long tons (171 t) of chlorine gas onto the battlefield . Though primarily a powerful irritant, it can asphyxiate in high concentrations or prolonged exposure . Being heavier than air, the gas crept across no man's land and drifted into the French trenches . The green - yellow cloud started killing some defenders and those in the rear fled in panic, creating an undefended 3.7 - mile (6 km) gap in the Allied line . The Germans were unprepared for the level of their success and lacked sufficient reserves to exploit the opening . Canadian troops on the right drew back their left flank and halted the German advance . The gas attack was repeated two days later and caused a 3.1 mi (5 km) withdrawal of the Franco - British line but the opportunity had been lost . </P> <P> The success of this attack would not be repeated, as the Allies countered by introducing gas masks and other countermeasures . An example of the success of these measures came a year later, on 27 April in the Gas attacks at Hulluch 40 km (25 mi) to the south of Ypres, where the 16th (Irish) Division withstood several German gas attacks . The British retaliated, developing their own chlorine gas and using it at the Battle of Loos in September 1915 . Fickle winds and inexperience led to more British casualties from the gas than German . French, British and German forces all escalated the use of gas attacks through the rest of the war, developing the more deadly phosgene gas in 1915, then the infamous mustard gas in 1917, which could linger for days and could kill slowly and painfully . Countermeasures also improved and the stalemate continued . </P> <P> Specialised aeroplanes for aerial combat were introduced in 1915 . Aircraft were already in use for scouting and on 1 April, the French pilot Roland Garros became the first to shoot down an enemy aircraft by using a machine - gun that shot forward through the propeller blades . This was achieved by crudely reinforcing the blades to deflect bullets . Several weeks later Garros force - landed behind German lines . His aeroplane was captured and sent to Dutch engineer Anthony Fokker, who soon produced a significant improvement, the interrupter gear, in which the machine gun is synchronised with the propeller so it fires in the intervals when the blades of the propeller are out of the line of fire . This advance was quickly ushered into service, in the Fokker E.I (Eindecker, or monoplane, Mark 1), the first single seat fighter aircraft to combine a reasonable maximum speed with an effective armament . Max Immelmann scored the first confirmed kill in an Eindecker on 1 August . Both sides developed improved weapons, engines, airframes and materials, until the end of the war . It also inaugurated the cult of the ace, the most famous being Manfred von Richthofen (the Red Baron). Contrary to the myth, anti-aircraft fire claimed more kills than fighters . </P>

Define western front what characterized this region during world war 1