From 1789, Nguyễn Ánh was once again in the ascendancy and began his northward march to defeat the Tây Sơn, reaching the border with the Qing dynasty by 1802, which had previously been under the control of the Trịnh lords.

Following their defeat, he succeeded in reuniting Vietnam after centuries of internecine feudal warfare, with a greater landmass than ever before, stretching from the Qing's borders down to the Gulf of Siam.

[16] Later, he fled to Hà Tiên on the southern coastal tip of Vietnam, where he met Pigneau de Behaine,[17][18][19] a French priest who became his advisor and played a major part in his rise to power.

[12][24] Pigneau hoped that by playing a substantial role in a Nguyễn Ánh victory, he would be in position to lever important concessions for the Catholic Church in Vietnam, helping its expansion in Southeast Asia.

Large parts of Nhon's supporters rebelled, weakening the Nguyễn army, and within a few months, the Tây Sơn had recaptured Saigon mainly on the back of naval barrages.

[41] The French administration in Pondicherry, led by acting governor Coutenceau des Algrains, was conservative in outlook and resolutely opposed intervention in southern Vietnam.

[43] In July 1786, after more than 12 months of fruitless lobbying in Pondicherry, Governor de Cossigny allowed Pigneau to travel back to France to directly ask the royal court for assistance.

[42][45] Pigneau told the court that if France invested in Nguyễn Ánh and acquired a few fortified positions on the Vietnamese coast in return, then they would have the capability to "dominate the seas of China and of the archipelago", and with it, control of Asian commerce.

[39][45] France promised four frigates, 1,650 fully equipped French soldiers and 250 Indian sepoys in return for the cession of Poulo-Condore (Côn Đảo) and Tourane (Da Nang),[39][42] as well as tree trade to the exclusion of all other countries.

[58][59] After more than a decade of conflict, Nguyễn Ánh had finally managed to gain control of Saigon for long enough to have time to start a permanent base in the area and prepare to build up for a decisive power struggle with the Tây Sơn.

[61] The French officers enlisted by Pigneau helped to train Nguyễn Ánh's armed forces and introduced Western technological expertise to the war effort.

[47][49][62] He introduced European infantry training, formations and tactics while also facilitating various methods of manufacturing and using European-style artillery, thereby making cannonry and projectiles a central part of the military.

[67] Following the construction of the citadel, the Tây Sơn never again attempted to sail down the Saigon River and try to recapture the city, its presence having endowed Nguyễn Ánh with a substantial psychological advantage over his opponents.

[68] Nguyễn Ánh took a keen personal interest in fortifications, ordering his French advisors to travel home and bring back books with the latest scientific and technical studies on the subject.

[25] Nguyễn Ánh used his new Chu Su Naval workshop to improve his inferior navy, which was much smaller than the Tây Sơn fleet and hitherto unable to prevent their rice raids.

Nguyễn Ánh studied naval carpentry techniques and was said to be adept at it, and learned navigational theory from the French books that Pigneau translated, particularly Denis Diderot and Jean le Rond d'Alembert's Encyclopédie.

[58][59] The majority of the fighting occurred in and around the coastal towns of Nha Trang in central Vietnam and Qui Nhơn further to the north in Bình Định Province, the birthplace and stronghold of the Tây Sơn.

[68] The large European wind-powered vessels gave the Nguyễn navy a commanding artillery advantage, as they had a superior range to the Tây Sơn cannons on the coast.

Combined with traditional galleys and a crew that was highly regarded for its discipline, skill and bravery,[76] the European-style vessels in the Nguyễn fleet inflicted hundreds of losses against the Tây Sơn in 1792 and 1793.

[76] In 1794, after a successful campaign in the Nha Trang region, Nguyễn Ánh ordered de Puymanel to build a citadel at Duyen Khanh, near the city, instead of retreating south with the seasonal northeasterly breeze.

[47][63][77] A common modern myth about this reign title is that was derived from Gia Định (Saigon) and Thăng Long (Hanoi) to symbolise the unification of northern and southern Vietnam, despite no contemporary evidence supporting this.

[79] After a quarter-century of continuous fighting, Gia Long had unified these formerly fractious territories, ultimately leading what is now modern Vietnam[63] and elevated his family to a position never previously occupied by any Vietnamese royalty.

The old patrimony of the Nguyễn formed the central part of the empire (vùng Kinh Kỳ), with nine provinces, five of which were directly ruled by Gia Long and his mandarins from Huế.

This system allowed Gia Long to reward his leading supporters with highly powerful positions, giving them almost total autonomy in ordinary administrative and legal matters.

[93][96] When Gia Long unified the country, it was described by Charles Maybon as being chaotic: "The wheels of administration were warped or no longer existed; the cadres of officials were empty, the hierarchy destroyed; taxes were not being collected, lists of communal property had disappeared, proprietary titles were lost, fields abandoned; roads bridges and public granaries had not been maintained; work in the mines had ceased.

[65][109][110] Recommendations from French officials in Pondicherry to Napoleon Bonaparte suggesting the re-establishment of diplomatic relations with Vietnam were fruitless due to the preoccupation with war in Europe.

The fortresses were built at Vinh, Thanh Hóa, Bắc Ninh, Hà Tĩnh, Thái Nguyên and Hải Dương in the north, Huế, Quảng Ngãi, Khánh Hòa and Bình Định in the centre, and Vĩnh Long in the Mekong Delta.

[117] Gia Long's fortifications program was marred by accusations that the people labored all day and part of the night in all weather conditions,[118] and that as a direct consequence, land went fallow.

This included restoring the civil service to the forefront of decision making, ahead of the army, and reversed Quang Trung's education reforms, which put science before the study of Confucian literature.

[110] Gia Long chose him for his strong character and his deep aversion to westerners, whereas Canh's lineage had converted to Catholicism and were reluctant to maintain their Confucian traditions such as ancestor worship.