linguistic:voice_change
Gia Long period

Nguyễn Phúc Ánh united Vietnam after a three-hundred-year division of the country. He celebrated his coronation at Huế on 1 June 1802 and proclaimed himself emperor (Vietnamese: Hoàng Đế), with the era name Gia Long (嘉隆). This title emphasized his rule from "Gia" Định region (modern-day Saigon) in the far south to Thăng "Long" (modern-day Hanoi) in the north. Gia Long prioritized the nation's defense and worked to avoid another civil war. He replaced the feudal system with a reformist Doctrine of the Mean, based on Confucianism. The Nguyen dynasty was founded as a tributary state of the Qing Empire, with Gia Long receiving an imperial pardon and recognition as the ruler of Vietnam from the Jiaqing Emperor for recognizing Chinese suzerainty. The envoys sent to China to acquire this recognition cited the ancient kingdom of Nanyue (Vietnamese: Nam Việt) to Emperor Jiaqing as the countries name, this displeased the emperor who was disconcerted by such pretentions, and Nguyễn Phúc Ánh had to officially rename his kingdom as Vietnam the next year to satisfy the emperor. The country was officially known as 'The (Great) Vietnamese state' (Vietnamese: Đại Việt Nam quốc),

Gia Long asserted that he was reviving the bureaucratic state that was built by King Lê Thánh Tông during the fifteenth-century golden age (1470–1497), as such he adopted a Confucian-bureaucratic government model, and sought unification with northern literati. To ensure stability over the unified kingdom, he placed two of his most loyal and Confucian-educated advisors, Nguyễn Văn Thành and Lê Văn Duyệt as viceroys of Hanoi and Saigon. From 1780 to 1820, roughly 300 Frenchmen served Gia Long's court as officials. Seeing the French influence in Vietnam with alarm, the British Empire sent two envoys to Gia Long in 1803 and 1804 to convince him to abandon his friendship with the French. In 1808, a British fleet led by William O'Bryen Drury mounted an attack on the Red River Delta, but was soon driven back by the Vietnamese navy and suffered several losses. After the Napoleonic War and Gia Long's death, the British Empire renewed relations with Vietnam in 1822. During his reign, a system of roads connecting Hanoi, Hue, and Saigon with postal stations and inns was established, several canals connecting the Mekong River to the Gulf of Siam were constructed and finished. In 1812, Gia Long issued the Gia Long Code, which was instituted based on the Ch'ing Code of China, replaced the previous Thánh Tông's 1480 Code. In 1811, a coup d'état broke out in the Kingdom of Cambodia, a Vietnamese tributary state, forcing the pro-Vietnamese King Ang Chan II to seek support from Vietnam. Gia Long sent 13,000 men to Cambodia, successfully restoring his vassal to his throne, and beginning a more formal occupation of the country for the next 30 years, while Siam seized northern Cambodia in 1814.

Gia Long died in 1819 and was succeeded by his fourth son, Nguyễn Phúc Đảm, who soon became known as Emperor Minh Mạng (r. 1820–1841) of Vietnam.