Sino-French War
Also called: Tonkin War
China and France fought over suzerainty in Vietnam. China lost its claim to Vietnam, and in the meantime the Gapsin Coup broke out in Korea.

Joseph Nash · Public domain · Source
Having taken southern Vietnam (Cochinchina) in the 1860s, France pushed into the north (Tonkin) and made Vietnam a protectorate in 1883. China, claiming suzerainty over Vietnam, sent troops to resist. In August 1884 the French fleet destroyed the Fujian Fleet — a product of the Self-Strengthening Movement — at Fuzhou in about an hour and also attacked Keelung in Taiwan. Chinese forces won at Zhennan Pass in March 1885, yet in June China signed a treaty at Tianjin recognising the French protectorate over Vietnam. France formed French Indochina in 1887. When China withdrew half its troops from Korea, radical reformers seized the moment to stage the Gapsin Coup (1884).