Secrecy was paramount. Britain's imperial general staff knew there would be outrage if it became known that the government was intending to use its secret stockpile of chemical weapons. But Winston Churchill, then secretary of state for war, brushed aside their concerns. As a long-term advocate of chemical warfare, he was determined to use them against the Russian Bolsheviks. In the summer of 1919, 94 years before the devastating strike in Syria, Churchill planned and executed a sustained chemical attack on northern Russia.

The British were no strangers to the use of chemical weapons. During the third battle of Gaza in 1917, General Edmund Allenby had fired 10,000 cans of asphyxiating gas at enemy positions, to limited effect. But in the final months of the first world war, scientists at the governmental laboratories at Porton in Wiltshire developed a far more devastating weapon: the top secret "M Device", an exploding shell containing a highly toxic gas called diphenylaminechloroarsine. The man in charge of developing it, Major General Charles Foulkes, called it "the most effective chemical weapon ever devised".

Trials at Porton suggested that it was indeed a terrible new weapon. Uncontrollable vomiting, coughing up blood and instant, crippling fatigue were the most common reactions. The overall head of chemical warfare production, Sir Keith Price, was convinced its use would lead to the rapid collapse of the Bolshevik regime. "If you got home only once with the gas you would find no more Bolshies this side of Vologda."The cabinet was hostile to the use of such weapons, much to Churchill's irritation. He also wanted to use M Devices against the rebellious tribes of northern India. "I am strongly in favour of using poisoned gas against uncivilised tribes," he declared in one secret memorandum. He criticised his colleagues for their "squeamishness", declaring that "the objections of the India Office to the use of gas against natives are unreasonable. Gas is a more merciful weapon than [the] high explosive shell, and compels an enemy to accept a decision with less loss of life than any other agency of war."

He ended his memo on a note of ill-placed black humour: "Why is it not fair for a British artilleryman to fire a shell which makes the said native sneeze?" he asked. "It is really too silly."