First large-scale use of poison gas in war

While the earliest documented use of a biological agent in war was in the 6th century BC, the first large-scale use of poison gas had to wait for World War I. The German Army used a form of tear gas, unsuccessfully, at the minor Battle of Bolimov on 31 January 1915, but the first major use of chlorine gas was during the Second Battle of Ypres, Belgium, between 22 April and 25 May 1915, when 168 tons of the poisonous gas were released over a 4-mile front. It was soon discovered, however, that the use of poison gas could be disadvantageous, because it was feared just as much by the German soldiers, as by the Allied Forces, against whom it was used.