There are many horrors of war, but perhaps one of the most horrible is chemical warfare. The use of gas as a weapon brings its own special dread. It is insidious, often silent, and causes an agonising death. It is a fairly modern invention: the first large scale poison gas attack took place near Ypres in southern Belgium, on 22nd April 1915.
This was not the first time that gas was used during the First World War. From the very start, both sides had experimented with irritant gases - basically, early forms of what we now call tear gas - to try to gain the advantage during an attack.
But this was the first time that the gas used was poisonous, that is, intended to kill rather than temporarily disable the enemy troops. The gas in question was chlorine, and over 160 tonnes of it were released by the German Army in one of the opening blows of the Second Battle of Ypres.
All that morning, the Germans had been heavily shelling the British and French lines around Ypres. The bombardment eased off in the afternoon but resumed again around 5pm. This time, the French and Algerian troops who held the line west of Ypres noticed a greenish-yellow cloud drifting towards them through the falling shells. Thinking the Germans were using smoke to mask an attack, they stood to arms, ready to defend their positions. But it wasn't smoke.
Chlorine gas has a strong, bleachy odour with a distinctive flavour of pepper and pineapples. When breathed in it mixes with the fluid in the lungs and turns into hydrochloric acid. As the French and Algerian troops - who had no protection at all against gas - started to choke and suffer with chest pains and breathing difficulties, panic broke out in their ranks. Those who still could fled in complete disorder, leaving a gap in their defences that was almost four miles wide.
This was, quite literally, more than the Germans could have wished for. They should have sent troops pouring through this enormous breach, but they didn't. Surprised by the scale of their success and reluctant to send their own men into a gas cloud, they advanced only slowly. Although they lost a good deal of ground, the nearby British and Canadian troops were able to stop the German advance. Had they failed, and had the key town of Ypres fallen into German hands, then the course of the war and, indeed, the course of history, might have been very different.