Various traders would visit Vietnam during the 18th century, until the major involvement of French forces under Pigneau de Béhaine from 1787 to 1789 helped establish the Nguyễn dynasty.

Alexandre de Rhodes returned to Europe in 1650, to advocate the dispatch of bishops in order to better accompany the development of Roman Catholicism in Vietnam (at that time around 100,000 converts), and the dispatch of bishops in order to create a strong native clergy and, to avoid in Vietnam a catastrophic eradication of Christianity, as seen in the case of Christianity in Japan around 1620:[1] "We have all reason to fear that what happened to the Church of Japan could also happen to the Church of Annam, because these kings, in Tonkin as well as in Cochinchina, are very powerful and accustomed to war...

It is necessary that the Holy See, by its own movement, give soldiers to these Oriental regions where Christians multiply in a marvelous way, lest, without bishops, these men die without sacrament and manifestly risk damnation.

"The efforts of Alexandre de Rhodes helped to the creation of the Paris Foreign Missions Society, marking the involvement of Catholic France as a new missionary power in Asia.

In 1658, Fathers Manoel Ferreira and Frenchman Joseph Tissanier arrived in Tonkin,[3] but they were expelled in 1664 under the rule of Trịnh Tạc, and fled to Ayutthaya.

[4] In June 1666, the Ayutthaya base of the Paris Foreign Missions Society dispatched Father François Deydier to Tonkin, who was able to reorganize Catholics there, although he remained in hiding.

[5] Mgr Lambert de la Motte himself would also visit the mission in Tonkin in 1669 and reinforce the organization there, under cover of trading activities of the French East India Company.

Towards the end of the 18th century, the Tây Sơn rebellion overthrew the Nguyễn family, but one of its members Nguyễn Ánh, future Emperor Gia Long, with the aid of the French Catholic priest Pigneau de Béhaine, titular bishop of Adran, obtained a treaty of alliance with the French king Louis XVI:[7] the Treaty of Versailles, signed on November 21, 1787.

In spite of these inconveniences, between 1789 and 1799, a French force mustered by Pigneau de Béhaine managed to support Gia Long in acquiring sway over the whole of Vietnam.

Several of these French adventurers would remain in high positions in the government of Gia Long such as Philippe Vannier, Jean-Baptiste Chaigneau, de Forsans and the doctor Despiau.

[11] In 1825, emperor Minh Mạng issued an edict prohibiting foreign missionaries in Vietnam, following the infiltration of Father Regéreau from the Thétis when it was anchored in Đà Nẵng.

"As the prohibition proved largely ineffective, and missionaries continued their activities in Vietnam, especially under the protection of the governor of Cochinchina Lê Văn Duyệt, a total ban on Roman Catholicism as well as French and Vietnamese priests was enacted following their support of the Lê Văn Khôi revolt (1833–1835), leading to persecutions of French missionaries and the execution of Father Joseph Marchand in 1835.

Following the defeat of China by Great Britain in the Opium War, emperor Minh Mạng attempted to build an alliance with European powers by sending a delegation under the mandarin Ton That Tuong in 1840.

Captain Favin-Lévêque, arriving in Đà Nẵng in 1840 on board the corvette Héroïne, obtained from Thiệu Trị the release of five imprisoned missionaries.

[11] In 1843, the French Foreign Minister, François Guizot, sent a fleet to the East under Admiral Jean-Baptiste Cécille and Captain Charner, together with the diplomat Lagrene.

[18] Rigault de Genouilly, with 14 French gunships, 3,000 men and 300 Filipino troops provided by the Spanish,[19] attacked the port of Đà Nẵng in 1858, causing significant damages, and occupying the city.

In 1866, France started the exploration of the Mekong river, with the objective of reaching the riches of China, under Ernest Doudart de Lagrée and Francis Garnier.

Again the Black Flags counter-attacked, and Rivière was killed in May 1883 in the Battle of Paper Bridge, leading to a huge movement in France in favour of a massive armed intervention.

China, the traditional overlord of Vietnam, kept contesting French influence in the area and was supporting Annam as well as the Black Flags on its territory at the frontier with Tonkin.

[29] Although a treaty had been signed between France and China (11 May 1884 the Tientsin Accord) promising Chinese evacuation from Tonkin, military confrontations continued as in the Bắc Lệ ambush (June 1884).

In 1945, Hồ Chí Minh declared an independent Democratic Republic of Vietnam, which was recognized by the fellow Communist governments of China and the Soviet Union.

Fighting lasted until March 1954, when the Việt Minh won the decisive victory against French forces at the grueling Battle of Điện Biên Phủ.

An explosion in the French mission complex across the Red River and 3 miles (4.8 km) southwest of the railway yard severely injured chief diplomat Pierre Susini, who later died of his wounds.