French assistance to Nguyễn Ánh French assistance to Nguyễn Ánh the future Emperor of Vietnam , whose name was later changed to Gia Long ) , the founder of the Nguyễn Dynasty , covered a period from 1777 to 1820 . From 1777 , Mgr Pigneau de Behaine , of the Paris Foreign Missions Society , had taken to protecting the young Vietnamese prince who was fleeing from the offensive of the Tây Sơn . Pigneau de Behaine went to France to obtain military aid , and obtained that a France - Vietnam alliance was signed through the 1787 Treaty of Versailles , between the king of France Louis XVI and Prince Nguyễn Ánh . As the French regime was under considerable strain at the eve of the French Revolution , France was unable to follow through with the application of the treaty . However , Mgr Pigneau de Behaine persisted in his efforts and with the support of French individuals and traders mounted a force of French soldiers and officers that would contribute to the modernization of the armies of Nguyễn Ánh , contributing to his victory and his reconquest of the totality of Vietnam by 1802 . A few French officers would remain in Vietnam after the victory , becoming prominent mandarins . The last of them left in 1824 , following the enthronement of Minh Mạng , Gia Long ' s successor . The terms of the 1787 Treaty of Alliance would still remain one of the justifications of French forces when they demanded the remittance of Đà Nẵng in 1847 . Protection of Nguyễn Ánh . The French first intervened in the dynastic battles of Vietnam in 1777 when 15 - year - old Prince Nguyễn Ánh , fleeing from an offensive of the Tây Sơn , received shelter from Mgr Pigneau de Behaine in the southern Principality of Hà Tiên . Pigneau de Behaine and his Catholic community in Hà Tiên then helped Nguyễn Ánh take refuge in the island of Pulo Panjang . These events created a strong bond between Nguyễn Ánh and Pigneau de Behaine , who took a role of protector over the young prince . Following this ordeal , Nguyễn Ánh was able to recapture Saigon in November 1777 and the whole of Cochinchina , and took the title of Commander in Chief in 1778 . Intervention in the Cambodian conflict ( 1780 - 1781 ) . In neighbouring Cambodia , a pro - Cochinchinese revolt erupted to topple the pro - Siam king Ang Non . In 1780 , the Cochinchinese troops of Nguyễn Ánh intervened , and Pigneau helped them obtain weapons from the Portuguese . The Bishop was even accused by the Portuguese of manufacturing weapons for the Cochinchinese , especially grenade s , a new weapon for Southeast Asia . Pigneau de Behaine also organized the supply of three Portuguese warships for Nguyễn Ánh . In his activities , Pigneau was supported by a French adventurer , Manuel . Contemporary witnesses clearly describe Pigneau ' s military role : 1782 - 1783 - 1785 Tây Sơn offensives . The French adventurer Manuel , in the service of Mgr Pigneau , took part in the battles against the 1782 offensive of the Tây Sơn . He fought commanding a warship against the Tây Sơn in the Saigon River , but he blew himself up with his warship rather than surrender to the more numerous Tây Sơn navy . In October 1782 , Nguyễn Ánh was able to recapture Saigon , only to be expelled again by the Tây Sơn in March 1783 . In March 1783 , the Nguyễn were again defeated , and Nguyễn Ánh and Pigneau fled to the island of Phú Quốc . They had to escape again when their hideout was discovered , being chased from island to island until they reached Siam . Pigneau de Behaine visited the Siamese court in Bangkok end 1783 . Nguyễn Ánh also reached Bangkok in February 1784 , where he obtained that an army would accompany him back to Vietnam . In January 1785 however , the Siamese fleet met with disaster against the Tây Sơn in the Mekong . Nguyễn Ánh again took refuge with the Siamese court , and again tried to obtain help from the Siamese . Nguyễn Ánh also resolved to obtain any help he could from Western countries . He asked Pigneau to appeal for French aid , and allowed Pigneau to take his five - year - old son Prince Cảnh with him . Pigneau also tried to obtain help from Manila , but the party of Dominicans he sent was captured by the Tây Sơn . From Pondicherry , he also sent a request for help to the Portuguese Senate in Macao , which would ultimately lead to the signature of a Treaty of Alliance between Nguyễn Ánh and the Portuguese on 18 December 1786 in Bangkok . Treaty of Versailles ( 1787 ) . Mgr Pigneau de Behaine arrived in Pondicherry with Prince Cảnh in February 1785 . The French administration in Pondicherry , led by the interim Governor Coutenceau des Algrains , successor of Bussy , seconded by Captain d ' Entrecasteaux , was resolutely opposed to intervening in southern Vietnam , stating that it was not in the national interest . In July 1786 , Pigneau was allowed to travel to France to ask the royal court directly for assistance . News of his activities reached Rome where he was denounced by the Spanish Franciscans , and offered Prince Cảnh and his political mandate to the Portuguese . They left Pondicherry for France in July 1786 . which they reached in February 1787 . Arriving in February 1787 with the child prince Canh at the court of Louis XVI in Versailles , Pigneau had difficulty in gathering support for a French expedition to install Nguyễn Ánh on the throne . This was due to the poor financial state of the country prior to the French Revolution . Eventually , he was able to seduce the technicians of military action with his precise instructions as to the conditions of warfare in Indochina and the equipment for the proposed campaign . He explained how France would be able to " dominate the seas of China and of the archipelago . " The party met with King Louis XVI , Minister of the Navy de Castries and Minister of Foreign Affairs Montmorin on May 5 or 6 , 1787 . Prince Cảnh created a sensation at the court of Louis XVI , leading the famous hairdresser Léonard to create a hairstyle in his honour " " au prince de Cochinchine " " . His portrait was made in France by Maupérin , and is now on display at the Séminaire des Missions Étrangères in Paris . Prince Cảnh dazzled the Court and even played with the son of Louis XVI , Louis - Joseph , Dauphin of France , who was about the same age . By November , his constant pressure had proved effective . On 21 November 1787 , the Treaty of Versailles was concluded between France and Cochin China in Nguyễn Ánh ' s name . Four frigates , 1650 fully equipped French soldiers and 250 Indian sepoy s were promised in return for Pulo Condore and harbour access at Tourane ( Da Nang ) . De Fresne was supposed to be the leader of the expedition . The French government , on the eave of the French Revolution , was in tremendous financial trouble , and saw its position weakened following the outbreak of civil war in Holland , theretofore a strategic ally in Asia . These elements strongly dampened its enthusiasm for Pigneau ' s plan between his arrival and the signature of the Treaty in November . A few days after the treaty was signed , the foreign minister sent instructions on 2 December 1787 to the Governor of Pondicherry Thomas Conway , which left the execution of the treaty to his own appreciation of the situation in Asia , stating that he was " free not to accomplish the expedition , or to delay it , according to his own opinion " Military assistance ( 1789 - 1802 ) . The party would leave France in December 1787 on board the " Dryade " , commanded by M . de Kersaint and accompanied by the " Pandour " , commanded by M . de Préville . They would again stay in Pondicherry from May 1788 to July 1789 . The " Dryade " was ordered by Conway to continue to Poulo Condor to meet with Nguyễn Ánh and deliver him 1 , 000 muskets bought in France and Father Paul Nghi , a Cochinchinese missionary devoted to Mgr Pigneau . However , Pigneau found the governor of Pondicherry unwilling to further fulfill the agreement . Although the Royal Council had already decided in October 1788 to endorse Conway , Pigneau was not informed until April . Pigneau was forced to use funds raised in France and enlist French volunteers . Pigneau was unaware of this duplicity . He defiantly noted : " I shall make the revolution in Cochinchina alone . " He rejected an offer from the English , and raised money from French merchants in the region . Conway finally provided two ships to Pigneau , the " Méduse " , commanded by Rosily , and another frigate to bring Pigneau back to Cochinchina . Pigneau used the funds he had accumulated to equip two more ships with weapons and ammunition , which he named the " Long phi " ( " " Le Dragon " " ) , commanded by Jean - Baptiste Chaigneau , and the " Phung phi " ( " " Le Phénix " " ) , commanded by Philippe Vannier , and he hired volunteers and deserters . Jean - Marie Dayot deserted the " Pandour " and was put in charge of supplies , transporting weapons and ammunitions on his ship the " St . Esprit " . Rosily , who had been commanding the " Méduse " deserted with 120 of his men , and was put in charge of recruitments . Pigneau ' s expedition left for Vietnam on June 19 , 1789 , and arrived at Vũng Tàu on 24 July 1789 . The forces gathered by Pigneau helped consolidate southern Vietnam and modernized its army , navy and fortifications . At the highest point , the total French military presence in Vietnam seems to have consisted in about 14 officers and about 80 men . Land forces . Olivier de Puymanel , a former officer of the " Dryade " who has deserted in Poulo Condor , built in 1790 the Citadel of Saigon and in 1793 the Citadel of Dien Khanh according to the principles of Vauban . He also trained Vietnamese troops in the modern use of artillery , and implemented European infantry methods in the Vietnamese army of Nguyễn Phúc Ánh . In 1791 , the French missionary Boisserand demonstrated to Nguyễn Ánh the usage of balloons and electricity . Puymanel suggested that these be used to bombard cities under siege such as Qui Nhơn , but Nguyễn Ánh refused to use these contraptions . In 1792 , Olivier de Puymanel was commanding an army of 600 men who had been trained with European techniques . Puymanel is said to have trained the 50 , 000 men of Nguyen ' s army . French bombs were used at the siege of Qui Nhơn in 1793 . From 1794 , Pigneau himself participated to all the campaigns , accompanying Prince Cảnh . He organized the defense of Dien Khanh when it was besieged by a vastly more numerous Tây Sơn army in 1794 . Navy . French Navy officers such as Jean - Marie Dayot and Jean - Baptiste Chaigneau were used to train the navy . By 1792 , a large Navy was built , with two European warships and 15 frigates of composite design . In 1792 , Dayot forced the harbour of Qui Nhơn , opening the way to the Cochinchinese fleet which then defeated the Tây Sơn fleet . In 1793 , Dayot led a raid in which 60 Tây Sơn galley s were destroyed . In 1799 , the Englishman Berry witnessed the departure of the Nguyễn fleet , composed of three sloops of war commanded by French officers , each of them with 300 men , 100 galleys with troops , 40 war junks , 200 smallers ships , and 800 transport boats . Jean - Marie Dayot also did considerable hydrographic work , making numerous maps of the Vietnamese coast , which were drawn by his talented brother . Arms trade . Nguyễn Ánh and Mgr Pigneau de Behaine also relied on French officers to obtain weapons and ammunitions throughout Asia through trade . In 1790 , Jean - Marie Dayot was sent by Nguyễn Ánh to Manila and Macao to trade for military supplies . Olivier de Puymanel , after having built several fortresses for Nguyễn Ánh , focused on the trading of weapons from 1795 . In 1795 and 1796 , he made two trips to Macao , where he sold Cochinchinese agricultural products in exchange for weapons and ammunitions . In 1795 he also travelled to Riau to trade rice received from Nguyễn Ánh . In 1797 - 98 , he travelled to Madras to obtain the remittance of the " Armida " , a warship belonging to Barizy , in the service of Nguyễn Ánh , which had been seized by the British in 1797 . Barizy , who had entered the service of Nguyễn Ánh in 1793 , also sailed to Malacca and Pulau Pinang to exchange Cochinchinese products against weapons . His ship , the " Armida " was captured by the British , but finally returned . In 1800 , Nguyễn Ánh sent him to trade with Madras to obtain weapons . According to one missionary , he was : Death of Pigneau de Behaine ( 1799 ) . Pigneau died at the siege of Qui Nhơn in October 1799 . Pigneau de Behaine was the object of several funeral orations on behalf of emperor Gia Long and his son Prince Cảnh . In a funeral oration dated 8 December 1799 , Gia Long praised Pigneau de Behaine ' s involvement in the defense of the country , as well as their personal friendship : The French forces in Vietnam continued the fight without him , until the complete victory of Nguyễn Ánh in 1802 . Qui Nhơn battle ( 1801 ) . The Tây Sơn suffered a major naval defeat at Qui Nhơn in February 1801 . The French took an active part in the battle . Chaigneau described the battle in a letter to his friend Barizy : On June 5 , 1801 , Nguyen left with his fleet for the north , and ten days later succeeded in capturing Huế . On 20 July 1802 , Nguyễn Ánh captured Hanoi and thus completed his reconquest of Vietnam . Continued French presence in Vietnam . Once Nguyễn Ánh became emperor Gia Long , several Frenchmen remained at the court to become mandarins , such as Jean - Baptiste Chaigneau . Chaigneau received the title of " truong co " , together with Philippe Vannier , de Forsans and Despiau , meaning second - class second - degree military mandarins , and later received the title of Grand Mandarin once Gia Long became emperor , with personal escorts of 50 soldiers . Several also married into a Vietnamese Catholic mandarin family , such as Chaigneau , Vannier or Laurent Barizy . The results of these French efforts at the modernization of Vietnamese forces were attested by John Crawfurd , who visited Huế in 1822 : With the death of Gia Long and the advent of Minh Mạng , relations strained considerably , and French advisors left the country . The last two of them , Jean - Baptiste Chaigneau and Philippe Vannier left Vietnam for France in 1824 , together with their Vietnamese families . Seclusion and persecutions . Only Catholic missionaries , mostly members of the Paris Foreign Missions Society , remained in Vietnam , although their activities were soon prohibited and they became persecuted . In Cochinchina , the Lê Văn Khôi revolt ( 1833 - 1835 ) united Vietnamese Catholics , missionaries and Chinese settlers in a major revolt against the ruling emperor , in which they were defeated . Persecutions would follow , leading to the killing of numerous missionaries such as Joseph Marchand in 1835 , Jean - Charles Cornay in 1837 , or Pierre Borie in 1838 , as well as local Catholics . In 1847 , French warships under Augustin de Lapierre and Charles Rigault de Genouilly demanded that persecutions cease , and that Da Nang be remitted to them in application of the 1787 Treaty of Versailles . The French sank the Vietnamese fleet in Da Nang in the Bombardment of Đà Nẵng ( 1847 ) , and negotiation with Emperor Thiệu Trị broke down . Persecutions of Catholics , combined with French desire for colonial expansion , would trigger ever stronger military interventions from France . The dispatch of an expeditionary force under Rigault de Genouilly would mark the return of the French military of Vietnamese soil , with the Siege of Đà Nẵng ( 1858 ) and the Capture of Saigon ( 1859 ) , origin of the establishment of French Indochina .