The Black Flag Army was a splinter remnant of a bandit group recruited largely from soldiers of ethnic Zhuang background, who crossed the border in 1865 from Guangxi, China into northern Vietnam, during the Nguyễn dynasty. Although brigands, they were known mainly for their fights against the invading French forces, who were then moving into Tonkin. The Black Flag Army is so named because of the preference of its commander, Liu Yongfu, for using black command flags.

Liu Yongfu was a Chinese warlord and commander of the celebrated Black Flag Army. Liu won fame as a Chinese patriot fighting against the French Empire in northern Vietnam (Tonkin) in the 1870s and early 1880s. During the Sino-French War, he established a close friendship with the Chinese statesman and general Tang Jingsong, and in 1895, he helped Tang organise resistance to the Japanese invasion of Taiwan. He succeeded Tang as the second and last president of the short-lived Republic of Formosa.