[3] The Vietnamese government, unable to confront Rivière with its own ramshackle army, enlisted the help of Liu Yongfu, whose well-trained and seasoned Black Flag soldiers were to prove a thorn in the side of the French.

In the summer of 1882 troops of the Chinese Yunnan and Guangxi Armies crossed the border into Tonkin, occupying Lạng Sơn, Bắc Ninh, Hưng Hóa and other towns.

On 27 March 1883, to secure his line of communications from Hanoi to the coast, Rivière captured the citadel of Nam Định with a flotilla of gunboats and a force of 520 French soldiers under his personal command.

On 27 and 28 March the commander of the Hanoi garrison, chef de bataillon Berthe de Villers, sallied out against Prince Hoang's Vietnamese army, around 6,000 strong, with two companies of marine infantry and a small force of sailors from the gunboat Léopard, leaving behind a single company of marine infantry to garrison the Royal Palace.

The senior Chinese civil mandarin Tang Jingsong reconciled the two men in April 1883, and persuaded Liu Yongfu to take the field against the French with the Black Flag Army.