World War I: The use of poison gas in World War I escalates when chlorine gas is released as a chemical weapon in the Second Battle of Ypres.

World War I, or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Major theatres of war included Europe and the Middle East, and well as parts of Africa and the Asia-Pacific. One of the deadliest conflicts in history,…