On the 22nd April 1915, the Second Battle of Ypres began in Belgium. The battle has become most remembered for seeing the first use of gas in the First World War, but Second Ypres really marked its first effective use. A form of tear gas had previously been used by the Germans fighting the Russians at Bolimov in the east 3 months earlier, but it had proved wholly unsuccessful. The freezing temperatures meant that a lot of the gas failed to vaporize, and that which did got blown back towards the German trenches.
At Ypres, the situation was dramatically different. 5,700 gas canisters were released by hand, all of which contained highly poisonous chlorine gas. The Germans relied on the wind to blow the gas towards their enemy but, despite some German casualties due to the rudimentary system of release, the gas was terribly effective.
Over 5,000 French Algerian, Moroccan and territorial troops died within ten minutes of the gas being released. A further 5,000 were temporarily blinded, with nearly half of them becoming prisoners of war.
The Germans didn’t expect the gas to be as effective as it was, and so didn’t fully exploit their initial advantage. However, by the end of the battle on 25th May, the Germans had certainly scored a tactical victory. They had compressed the size of the Ypres salient and had demonstrated the effectiveness of chemical warfare. The Allies soon developed their own poison gas, making chemical warfare part of the offensive strategy for the rest of the war.