The 2nd Battle of Ypres took place from April 22 – May 25, 1915 between the French and the Germans.  In this battle we see the introduction of poison gas as a weapon. 168 tons of chlorine gas was released by the Germans along a front that was nearly 7 kilometers wide.  Within minutes the French had suffered around 6,000 casualties.  Poison gas was a reaction to the slow stalemates that resulted from trench warfare.  It was an indirect means of attacking that could be launched from a distance.  It developed into different kinds: chlorine gas, tear gas, phosgene gas, and mustard gas.  Masks would help deter all of these except for mustard gas because of its heavy nature and ability to sink into crevices and stay there for weeks.  Poison gas became soldiers’ greatest fear, and this was the most difficult technological advancement in the war for trench design to specifically respond to.  More technology was needed to create and counter gas and that is a prime example of how impactful technology and weaponry had become in this time period.