Prohibitions on specific types of weapons can sometimes arise very quickly in international law, and with universal effect. On 22 April 1915, the German Army released 168 tonnes of chlorine gas near the Belgian city of Ypres. 1 Five thousand soldiers died in the Allied trenches that day while another 10,000 were grievously injured. Three months later, the British Army launched its own first chlorine gas attack. By the end of the First World War, chemical weapons had killed nearly 100,000 people and wounded an estimated one million. 2 After the war, these horrors prompted the negotiation of the 1925 Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or Other Gases, and of Bacteriological Methods of Warfare. 3 Today, the prohibition on the use of chemical weapons is regarded as a jus cogens rulea customary international law 'taboo' that tolerates no exceptions, not even exceptions created by way of treaty. 4 Compliance with the rule has not been perfect: Saddam Hussein used mustard gas against Iranian forces in the 1980s and then against Kurdish civilians in northern Iraq. 5 The international community responded with the 1992 Convention on the Development, Production, Stockpiling and 1 David Hughes, 'Chemical weapons: The day the first poison gas attack changed the face of warfare forever', The Independent (28 April 2016), online: www.independent.co.uk/news/ world/politics/chemical-weapons-warfare-remembrance-day-poison-mustard-gas-firstworld-war-ypres-isis-a7005416.html. 2 Ibid. 3 Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or Other Gases, and of Bacteriological Methods of Warfare, 17 June 1925, 94 LNTS 65 (entered into force 9 May 1926).