Haber greeted World War I with enthusiasm, joining 92 other German intellectuals in signing the "Manifesto of the Ninety-Three" in October 1914. Haber played a major role in the development of the non-ballistic use of chemical warfare in World War I, in spite of the proscription of their use in shells by the Hague Convention of 1907 (to which Germany was a signatory). He was promoted to the rank of captain and made head of the Chemistry Section in the Ministry of War soon after the war began. In addition to leading the teams developing chlorine gas and other deadly gases for use in trench warfare, Haber was on hand personally when it was first released by the German military at the Second Battle of Ypres (22 April to 25 May 1915) in Belgium. Haber also helped to develop gas masks with adsorbent filters which could protect against such weapons.