By 1857 Louis-Napoleon had been persuaded that invasion was the best course of action, and French warships were instructed to take Tourane without any further efforts to negotiate with the Vietnamese. Tourane was captured in late 1858 and Gia Dinh (Saigon and later Ho Chi Minh City) in early 1859. In both cases Vietnamese Christian support for the French, predicted by the missionaries, failed to materialize. Vietnamese resistance and outbreaks of cholera and typhoid forced the French to abandon Tourane in early 1860. Meanwhile, fear was growing in Paris that if France withdrew the British would move in. Also current in Paris at that time was the rationalization that France had a civilizing mission--a duty to bring the benefits of its superior culture to the less fortunate lands of Asia and Africa. (This was a common justification for the colonial policies of most of the Western countries.) Meanwhile, French business and military interests increased their pressure on the government for decisive action. Thus in early 1861, a French fleet of 70 ships and 3,500 men reinforced Gia Dinh and, in a series of bloody battles, gained control of the surrounding provinces. In June 1862, Emperor Tu Duc, signed the Treaty of Saigon agreeing to French demands for the cession of three provinces around Gia Dinh (which the French had renamed Saigon) and Poulo Condore, as well as for the opening of three ports to trade, free passage of French warships up the Mekong to Cambodia, freedom of action for the missionaries, and payment of a large indemnity to France for its losses in attacking Vietnam.
Even the French were surprised by the ease with which the Vietnamese agreed to the humiliating treaty. Why, after successfully resisting invasions by the Chinese for the previous 900 years, did the monarchy give in so readily to French demands? Aside from the seriousness of the loss of Saigon and the possible overestimation of French strength, it appears that the isolation of the monarchy from the people created by decades of repression prevented Tu Duc and his court from attempting to rally the necessary popular support to drive out the French. In fact, by placating the French in the south, Tu Duc hoped to free his forces to put down a widespread Christian-supported rebellion in Bac Bo, which he indeed crushed by 1865. French missionaries, who had urged their government to support this rebellion, were disillusioned when it did not, especially after thousands of Christians were slaughtered by Tu Duc's forces following the rebellion. The missionaries, however, had served only as an initial excuse for French intervention in Vietnam; military and economic interests soon became the primary reasons for remaining there.
The French navy was in the forefront of the conquest of Indochina. In 1863 Admiral de la Grandiere, the governor of Cochinchina (as the French renamed Nam Bo), 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 completed the annexation of Cochinchina by seizing the remaining three western provinces. The following month, the Siamese government agreed 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 turned to Tonkin (as the French referred to Bac Bo). The 1873 storming of the citadel of Hanoi, led by French naval officer Francis Garnier, had the desired effect of forcing Tu Duc to sign a treaty with France in March 1874 that recognized France's "full and entire sovereignty" over Cochinchina, and opened the Red River to commerce. In an attempt to secure Tonkin, Garnier was killed and his forces defeated in a battle with Vietnamese regulars and Black Flag forces. The latter were 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.
In April 1882, a French force again stormed the citadel of Hanoi, under the leadership of naval officer Henri Riviere. Riviere and part of his forces were wiped out in a battle with a Vietnamese-Black Flag army, a reminder of Garnier's fate a decade earlier. While Garnier's defeat had led to a partial French withdrawal from Tonkin, Riviere's loss strengthened the resolve of the French government to establish a protectorate by military force. Accordingly, additional funds were appropriated by the French Parliament to support further military operations, and Hue fell to the French in August 1883, following the death of Tu Duc the previous month. A Treaty of Protectorate, signed at the August 1883 Harmand Convention, established a French protectorate over North and Central Vietnam and formally ended Vietnam's independence. In June 1884, Vietnamese scholar-officials were forced to sign the Treaty of Hue, which confirmed the Harmand Convention agreement. By the end of 1884, there were 16,500 French troops in Vietnam. Resistance to French control, however, continued. A rebellion known as the Can Vuong (Loyalty to the King) movement formed in 1885 around the deposed Emperor Ham Nghi and attracted support from both scholars and peasants. The rebellion was essentially subdued with the capture and exile of Ham Nghi in 1888. Scholar and patriot Phan Dinh Phung continued to lead the resistance until his death in 1895. Although unsuccessful in driving out the French, the Can Vuong movement, with its heroes and patriots, laid important groundwork for future Vietnamese independence movements.