The Black Flag Army (Chinese: 黑旗軍; pinyin: Hēiqí Jūn; Vietnamese: Quân cờ đen, chữ Nôm: 軍旗黰) was a splinter remnant of a bandit and mercenary group recruited largely from soldiers of ethnic Zhuang background and former Taiping soldiers who crossed the border in 1865 from Guangxi, China into northern Vietnam, during the Nguyễn dynasty, and were hired and sponsored by Vietnamese authorities to fight against other bandits and rebels.

[2] The harassment of European vessels trading on the Red River triggered the dispatch of the French expeditionary force to Tonkin under Commandant Henri Rivière in 1882.

The Black Flag Army formally disbanded at the end of the Sino-French War, though many of its members continued to harass the French for years afterwards as freelance bandits.

Liu Yongfu crossed to Taiwan at the appeal of his old friend Tang Jingsong, the island's former governor-general and now president of the short-lived Republic of Formosa.

In 1865 the rebels, who had retired before the Chinese troops into the province of Kwangsi, were finally driven across the border into Tonquin, and found a secure retreat in the mountains on both sides of the Red river valley.

The main body of the original Taeping rebels were disposed to settle down to peaceful pursuits, and to make their submission to the Chinese and Anamese authorities.

It was through a want of precaution against the skill aud courage of the Black Flags that Garnier lost his life in 1873, and through a repetition of the same blunder Riviera sufiered defeat and death in 1883.

"[3]In 1873, the Vietnamese government enlisted the help of the Black Flag Army to face an attempt to conquer Tonkin by French naval lieutenant Francis Garnier, who was acting without orders after having been sent there on a diplomatic mission.

The youthful enseigne de vaisseau Adrien-Paul Balny d’Avricourt led a similar small column to reinforce Garnier but was also killed in front of his men.

[4] Despite Garnier's death, the attempt to retake Hanoi had failed and the French remained in control of the greater part of the Red River Delta.

[5] However, the French government disapproved the unauthorized conquest and lieutenant Paul Philastre was sent to remove Garnier's men from the cities they occupied and repatriate them back to Saigon in February 1874.

[9] In December 1883, the Black Flag Army suffered a major defeat at the hands of Admiral Amédée Courbet in the Sơn Tây Campaign.

Even with large Chinese and Vietnamese regular contingents at Sơn Tây, the Black Flag Army bore the brunt of the fighting, and took very heavy casualties.

On the evening of 11 April, seeing Brière de l'Isle's Turcos and marine infantry emerging behind their flank at Xuân Đông, the Black Flags evacuated Hưng Hóa before they were trapped inside it.

[11] The Black Flag Army retreated up the Red River to Thanh Quan, only a few days march from the frontier town of Lào Cai.

One of Millot's final achievements was to advance up the Lô River and throw the Black Flag Army out of Tuyên Quang in the first week of June, again without a single French casualty.

But French attention was diverted by the sudden crisis with China provoked by the Bắc Lệ ambush (23 June 1884), and during the eventful summer of 1884 the Black Flags were left to lick their wounds.

The Empress Dowager Cixi responded to the news of the destruction of China's Fujian Fleet at the Battle of Fuzhou (23 August 1884) by ordering her generals to invade Tonkin to throw the French out of Hanoi.

The Black Flags helped the Chinese forces put pressure on Hưng Hóa and the isolated French posts of Phủ Doãn and Tuyên Quang during the autumn of 1884.

Unpaid for months and still in possession of their rifles, most of the unwanted Black Flag soldiers immediately took to banditry, under cover of the Cần Vương resistance movement against the French.