What gas was first used at Ypres in 1915?

The first gas used at Ypres in 1915 was chlorine gas. Employed by the German army during the Second Battle of Ypres, it marked one of the first large-scale uses of chemical warfare in World War I. The gas caused severe respiratory issues and panic among the Allied troops, resulting in significant casualties. This attack led to the development of gas masks and other protective measures in response to the threat of chemical weapons.