100 years ago, 12–13 July, 1917, mustard gas was first used. Chemical warfare is prohibited by the Geneva Protocol of 1925 and the Chemical Weapons Convention of 1993. It was designed to render ground uninhabitable.
Read more at the Simon Jones Historian blog, "Yellow Cross: the advent of mustard Gas in 1917."
Image (click to enlarge), words of "Nurse S. Millard". Van Bergen, Leo. Before My Helpless Sight: Suffering, Dying and Military Medicine on the Western Front, 1914–1918. 2009. Ashgate Publishing, Ltd. p. 184. ISBN 978-0-7546-5853-5; citing Winter, Denis. Death's Men: Soldiers Of The Great War. 1985. Penguin Books. ISBN-13: 978-0140168228.
Image (click to enlarge), "The first mustard gas bombardment reported in the War Diary of the 45th Infantry Brigade, 12 July 1917. (The National Archives, WO95/1943)"; Simon Jones.