Montigny Mission
In October 1855, Emperor Napoleon III of the Second French Empire appointed Charles de Montigny as the French imperial plenipotentiary to negotiate for a new commercial treaty with the Kingdom of Siam, to remonstrate the issue of Vietnamese persecution of Catholics and to conclude a new treaty with Nguyen-dynasty Vietnam. Montigny arrived in Siam, in Bangkok, in July 1856 during the reign of King Mongkut of Siam, where he successfully concluded the Franco-Siamese Treaty of August 1856. After staying in Siam for two months, Montigny left for Cambodia in September 1856 where, due to many obstacles and the Siamese intervention, Montigny was unable to meet King Ang Duong of Cambodia at Oudong. During his stay in Kampot, Montigny preliminarily sent two French corvettes to Vietnam, during the reign of Emperor Tự Đức. Vietnam's stern refusal to receive the French diplomatic letter resulted in the French bombardment of Danang in September 1856. After many unsettling issues, Montigny did not manage to conclude a treaty with Cambodia and was obliged to left for Vietnam, where Montigny arrived at Danang (also called Tourane) in January 1857. Montigny's negotiation with the Vietnamese imperial court also resulted in failure. Montigny left Vietnam with contempt, threatening Vietnam with military intervention. Montigny's diplomatic mission to three Southeast Asian polities, namely Siam, Cambodia and Vietnam, from mid-1856 to early 1857, marked the advent of impactful French presence in the region, leading to the French conquest of Cochinchina and Franco–Siamese dispute over Cambodia in the 1860s.
At the conclusion of the Siamese–Vietnamese War in 1847, King Harireak Reamea Ang Duong was enthroned with both Siamese and Vietnamese endorsements as Ang Duong was obliged to pay tributes to both Siam and Vietnam. The Cambodian king Ang Duong then sought for an external Western power to protect Cambodia from both Siam and Vietnam. Bishop Jean-Claude Miche the Apostolic Vicar of Cambodia, on behalf of Ang Duong, wrote a letter to Gauthier the French Consul at Singapore, proposing to establish the Franco–Cambodian relation. The letter reached Singapore in December 1852. Gauthier the French Consul, however, was not committed to the cause. Sir John Bowring the British plenipotentiary concluded the Bowring Treaty with Siam in April 1855. In October 1855, Charles de Montigny was appointed as the French plenipotentiary to conclude a treaty with Siam in similar manner with the British. Montigny reached Singapore in May 1856, where he was assigned additional missions to deal with the Vietnamese persecution of Catholics, to conclude a similar commercial treaty with Vietnam and also to inform King Ang Duong of Cambodia that his gifts had not yet reached France. Siamese government, who regarded Cambodia as its vassal, was largely unaware of this discreet Cambodian contact with France.
Montigny arrived in Bangkok in July 1856 with French corvettes Marceau, Catinat and Capricieuse. Montigny was received by the Siamese royal court in grand ceremonies at the Siamese royal palace. During Montigny's negotiations in Siam, Montigny asked the Siamese king Mongkut about Cambodia and asked for Khmer translators. The Siamese then realized that Cambodia had been in confidential contact with France and sought to prevent Cambodia from establishing relations with France. Montigny also sent corvette Catinat under the command of Le Lieur to Vietnam for survey. The Franco–Siamese "Montigny Treaty" was successfully concluded at Bangkok on 15 August 1856.
French corvette Catinat under Le Lieur reached Danang in September 1856. Le Lieur sent a French priest Charles Fontaine to present a French diplomatic letter to the Vietnamese authorities at Danang, who refused to receive the letter. Le Lieur then propelled Catinat into the Perfume River to present the letter to the Vietnamese at Huế, where the Vietnamese Emperor resided. The Vietnamese again refused to take the letter, leaving the letter on the riverbank. Enraged, Le Lieur delivered an ultimatum. As Vietnam remained unresponsive, Le Lieur bombarded Danang with cannons from Catinat. On 26 September 1856, Le Lieur led French marines in an amphibious assault on Danang. The Vietnamese in Danang suffered damages and casualties in this French attack. Emperor Tự Đức responded by organizing defenses under the Vietnamese commander Đào Trí.
After the conclusion of the treaty with Siam, Montigny left Siam for Cambodia in September 1856, taking some Khmer men and the translators with him on Capricieuse to Cambodia. At his own initiative, Montigny intended to make a treaty with Cambodia. However, among the passengers were three Siamese spies. Upon Montigny disembarkation at Kampot in October, the Siamese agents coerced the Cambodian officials in Kampot not to agree to any French proposals. Bishop Jean-Claude Miche helped Montigny uncovered these Siamese agents, who were chased away by Montigny. Montigny also sent corvette Capricieuse under Collier to join with Le Lieur in Vietnam. Montigny demanded that the Cambodian king Ang Duong come to meet him at Kampot to conclude a treaty. Ang Duong, in turn, invited Montigny to take the road from Kampot to Oudong to meet him. However, Montigny was unwilling to go to Oudong as he was late in his due arrival in Vietnam, where his main objective called. Totally unaware of the connoted Cambodian wish for French protection, Montigny assigned a young French priest Hertrest to bring his draft treaty for Ang Duong to sign at Oudong, while Montigny himself left Cambodia with Bishop Miche to Vietnam.
The French priest Hertrest presented Montigny's draft treaty to King Ang Duong at Oudong in an audience with the presence of the pro-Siamese Battambang officials, who closely monitored Ang Duong's interactions with the French. Montigny's draft treaty only contained the commercial and religious issues and said nothing about French protection over Cambodia. Ang Duong rejected this treaty, which served him no purposes other than exposing his secret endeavors with France to Siam. Siam thus successfully prevented Cambodia from establishing relations with France for a time being.
Collier, with his corvette Capricieuse, joined his colleague Le Lieur at Danang in late October 1856. Both Collier and Le Lieur attempted to present diplomatic letter to Vietnam again but to no avail. The outbreak of the Second Opium War in October 1856 compelled both Collier and Le Lieur to take their respective corvettes Capricieuse and Catinat to leave Vietnam for China. The Vietnamese Emperor Tự Đức claimed this 'victory' over the French invaders. Due to the strong opposing northwestern monsoon winds, Montigny, from Cambodia, was obliged to take a circumventing detour, having to go through the coast of Borneo to Manila in the Philippines in order to reach Danang. Montigny and Bishop Miche eventually arrived in Danang in January 1857, three months after the departure of Collier and Le Lieur. Montigny threatened the Vietnam with gunboat diplomacy. Emperor Tự Đức appointed Đào Trí as the Vietnamese plenipotentiary to negotiate with Montigny. The negotiations apparently failed as both Montigny and Đào Trí did not find a compromising ground. Unlike Siam, Vietnam was unwilling to sign an unequal treaty.