Chemical weapons use the toxic properties of chemical substances to produce physical or physiological effects on an enemy. These weapons come in many forms such as nerve agents that affect how the body works, or physical agents such as mustard gas which caused painful blisters internally and externally.
Prior to World War I, it was considered uncivilized to develop and use chemical weapons. The demand and popularity for these weapons came because of the stalemate that occurred during in-trench fighting.
Although it's believed that Germany was the first to use chemical agents in WWI, it was actually the French who fired tear gas grenades within the first month of War.
The Germans were the first to give serious study and develop new chemical weapons.
The first use of poison gas came on April 22, 1915, at the start of the Second Battle of Ypres. Chlorine, within seconds of inhaling its vapor it destroyed the victim's respiratory organs, brought on choking attacks. During that day, Germans poured heavy amounts of chlorine around the area of French troops. The yellow-green cloud drifted toward the trench and caused panic and devastation. This was the first milestone chemical weapons had come across. Now the whole world knew of the weapons the German army had, and began to consider chemicals for their own country.
The attack had one clear benefit for Germany. It brought to an end German hesitancy and disagreement over its use, and the use of poison gas continued to escalate for the remainder of the war.
After the use of chlorine gas came the use of phosgene. Phosgene as a weapon was more potent than chlorine in that phosgene caused much less coughing with the result that more of it was inhaled. It took 48 hours for a healthy soldier to be taken down after inhaling the gas. Both the German and Allied armies quickly adopted this weapon.
Germany remained ahead in the development of gases, and unveiled an advanced new gas that was hard to protect against; mustard gas.