The French navy is in the forefront…

The French navy is in the forefront of the conquest of Indochina.

In 1863 Admiral de la Grandiere, the governor of Cochinchina (as the French have renamed Nam Bo), had forced the Cambodian king to accept a French protectorate over that country, claiming that the Treaty of Saigon had made France heir to Vietnamese claims in Cambodia.

In June 1867, the admiral completes he annexation of Cochinchina by seizing the remaining three western provinces.

The following month, the Siamese government agrees to recognize a French protectorate over Cambodia in return for the cession of two Cambodian provinces, Angkor and Battambang, to Siam.

With Cochinchina secured, French naval and mercantile interests turn to Tonkin (as the French refer  to Bac Bo).

The 1873 storming of the citadel of Hanoi, led by French naval officer Francis Gamier, has the desired effect of forcing Tu Duc to sign a treaty with France in March 1874 that recognizes France's "full and entire sovereignty" over Cochinchina, and opens the Red River to commerce.

In an attempt to secure Tonkin, Gamier is killed and his forces defeated in a battle with Vietnamese regulars and Black Flag forces.

The latter are Chinese soldiers who had fled south following the Taiping Rebellion in that country and had been hired by the Hue court to keep order in Tonkin.