In the mid-1870s and 1880s Giquel became increasingly involved in international diplomacy. He first served as an adviser during the "Taiwan crisis", a diplomatic clash between Japan and China in 1874 culminating with Japan's invasion of the island. In 1881 he helped Tseng Chi-tse peacefully conclude the "III crisis" between China and Russia. Giquel spent his last years (1883–1885) struggling to help end the Sino-French War, which had broken out due to conflicting Sino-French claims to Indochina. Among the traumatic events of that period, certainly for Prosper Giquel, was the August 1884 destruction by the French Navy of the Fuzhou Dockyard, the principal accomplishment of his entire career in China, in the Battle of Fuzhou.