Before the end of Second World War, French Indo-China comprised five separate countries : Tonkin, Annam, Cochinchina, Laos and Cambodia. The first three are inhabited by a people popularly known as “Annamite” now (Vietnamese) whose culture derives from China, while Cambodia, with a civilization which is Indian in origin, is clearly distinguished from the three countries .
The Annamites are descendants of the Viets, one of the autochthonous races of South China, who have emigrated southward in large numbers and spread the selves like “a flood-ware or a patch of oil from the Red River to the lower Mekong, and from the Chinese Frontier and the Gulf of Tonkin to Camau Point and the Gulf of Siam”. (Histoire des Pays de l’Union Indochinoise by NGUYEN MANH QUE - Page 24). They destroyed the kingdom of Champa (a country inhabited by the Malays) and in February, 1859 (the date of capture of Saigon), they occupied a part of Cambodian territory at the moment of the establishment of the French in Indo-China. This occupation led to irregular cession of land by Cambodian rebel princes and also to acts of generosity on the part of Cambodian sovereigns.... For example, the Srok of Preah Trapeang (Travinh) offered asylum to the Emperor GIALONG who had been expelled from his country by the revolt of the TAY SON. The Emperor GIALONG organized his Army there and obtained military help from the Cambodia King ANG ENG (who reigned from 1779 to 1796). After the revolt of the TAY SON had been repressed and the Emperor had returned to the throne of Annam, he recalled the kindly hospitality of the province of Travinh and asked ANG ENG to exclude this Srok from all taxes and the inhabitants from all corvées. ANG ENG agreed out of friendship for Emperor. Later GIALONG arbitrarily made this Srok an Annamite colony, (cf. A letter from ANG DUONG to the Emperor NAPOLEON III quoted in the History of Thailand).
In the same way, the region Saigon. Bienhoa and Baria opened to Annamite immigration by the authority of King CHEY CHETTHA II, granted in 1663. The Annamite Prince NGUYEN SAI VUONG asked that, in return for the payment of taxes, the immigrants should have the right to cultivate these lands and trade there. King CHEY CHETTHA, who had married an Annamite Princess, daughter of NGUYEN SAI VUONG agreed since, according to the custom of the Khmer Empire, the Queen, the Dowager Queen and the Viceroy possessed certain personal prerogatives for life over certain provinces of the Empire. These provinces were always the property of the crown but the person enjoying the apanage was entitled to certain administrative rights (in matters of taxation, police, etc.) over the territory under his authority. It was this concession which allowed the Annamites to get their feed on Cambodia territory for the first time.
In 1853, faced with Annamite expansion on Cambodia territory and fearing an alliance between Siam and Annam for the partition of Cambodia, King ANG DUONG secretly sent a letter together with various gifts to the French Consul in Singapore. The letter was addressed to the Emperor NAPOLEON III and asked for the protection of France for Cambodia (Histoire des Pays de l’Union Indochinoise by NGUYEN MANH QUE - page 280 and Histoire sommaire du Royaume du Cambodge by HENRI RUSSIER - page 83). No reply to this letter was received and ANG DUONG declined to send a further letter to France with new gifts, proposing to France the conclusion of an alliance with Cambodia and begging the French Emperor not to accept certain territories specified in the letter, if Annam were to cede them to France, because these territories belonged to Cambodia, (Cf. A letter from ANG DUONG quoted in the History of Thailand.)
Stirred by the different impulses which let to expansion in the 19th century and in particular impressed by the King of Cambodia’s tokens of confidence and friendliness, France intervened in Cochinchina. In the attack on Saigon in 1859, Cambodia troops supported the French by advancing at the same time into the provinces of Meat Chruk (Chaudoc). Kramuon Sar (Rachgia). Srok de Treang (Soctrang). Preah Trapreang (Travinh).
By the Treaty of Peace and Friendship, signed at Saigon on the 5th June, 1862, Annam accepted (with certain clauses relating to the free exercise of Catholic religion on her territory and an engagement not to cede any part her territory whatsoever to anyone without consulting France, the opening of Franco-Spanish trade in certain parts and the payment of a war indemnity) a clause which particularly concerned Cambodia since it was by virtue of this clause that Annam ceded to France three provinces occupied by the Annamites ; Bienhoa, Giadinh and Mytho.
This clause is therefore not valid since Annam was ceding to a third party possessions which did not belong to her.Subsequently in 1867, the representative of France Admiral LAGRANDIERE, under the pretext of Annam’s violation of the Treaty of Saigon, took Vinhlong, Chaudoc and Hatien, which are three other Cambodian provinces, and occupied the whole of Western Cochinchina. Finally, France’s conquest of the Isle of Phuquoc, likewise Cambodian territory, made the whole of Cochinchina now South Viet-Nam, a French Colony formally established by the Franco-Annam Treaty of 1874. ( Histoire des Pays de l’Union Indochinoise by NGUYEN MANH QUE - pages 164 to 167).