The Lê dynasty was overthrown by its general named Mạc Đăng Dung in 1527. He killed the Lê emperor and proclaimed himself emperor, starting the Mạc Dynasty. After defeating many revolutions for two years, Mạc Đăng Dung adopted the Trần Dynasty's practice and ceded the throne to his son, Mạc Đăng Doanh, who became Thái Thượng Hoàng.
Meanwhile, Nguyễn Kim, a former official in the Lê court, revolted against the Mạc and helped king Lê Trang Tông restore the Lê court in the Thanh Hóa area. Thus a civil war began between the Northern Court (Mạc) and the Southern Court (Restored Lê). Nguyễn Kim's side controlled the southern part of Đại Việt (from Thanhhoa to the south), leaving the north (including Đông Kinh-Hanoi) under Mạc control. When Nguyễn Kim was assassinated in 1545, military power fell into the hands of his son-in-law, Trịnh Kiểm. In 1558, Nguyễn Kim's son, Nguyễn Hoàng, suspecting that Trịnh Kiểm might kill him as he had done to his brother to secure power, asked to be governor of the far south provinces around present-day Quảng Bình to Bình Định. Hoang pretended to be insane, so Kiem was fooled into thinking that sending Hoang south was a good move as Hoang would be quickly killed in the lawless border regions. However, Hoang governed the south effectively while Trịnh Kiểm, and then his son Trịnh Tùng, carried on the war against the Mạc. Nguyễn Hoàng sent money and soldiers north to help the war but gradually he became more and more independent, transforming their realm's economic fortunes by turning it into an international trading post.
The civil war between the Lê/Trịnh and Mạc dynasties ended in 1592, when the army of Trịnh Tùng conquered Hanoi and executed king Mạc Mậu Hợp. Survivors of the Mạc royal family fled to the northern mountains in the province of Cao Bằng and continued to rule there until 1667 when Trịnh Tạc conquered this last Mạc territory. The Lê kings, ever since Nguyễn Kim's restoration, only acted as figureheads. After the fall of the Mạc Dynasty, all real power in the north belonged to the Trịnh Lords.
In the year 1600, Nguyễn Hoàng also declared himself Lord (officially "Vương", popularly "Chúa") and refused to send more money or soldiers to help the Trịnh. He also moved his capital to Phú Xuân, modern-day Huế. Nguyễn Hoàng died in 1613 after having ruled the south for 55 years. He was succeeded by his 6th son, Nguyễn Phúc Nguyên, who likewise refused to acknowledge the power of the Trịnh, yet still pledged allegiance to the Lê king.
Trịnh Tráng succeeded Trịnh Tùng, his father, upon his death in 1623. Tráng ordered Nguyễn Phúc Nguyên to submit to his authority. The order was refused twice. In 1627, Trịnh Tráng sent 150,000 troops southward in an unsuccessful military campaign. The Trịnh were much stronger, with a larger population, economy and army, but they were unable to vanquish the Nguyễn, who had built two defensive stone walls and invested in Portuguese artillery.
The Trịnh-Nguyễn War lasted from 1627 until 1672. The Trịnh army staged at least seven offensives, all of which failed to capture Phú Xuân. For a time, starting in 1651, the Nguyễn themselves went on the offensive and attacked parts of Trịnh territory. However, the Trịnh, under a new leader, Trịnh Tạc, forced the Nguyễn back by 1655. After one last offensive in 1672, Trịnh Tạc agreed to a truce with the Nguyễn Lord Nguyễn Phúc Tần. The country was effectively divided in two.
The Trịnh and the Nguyễn maintained a relative peace for the next hundred years, during which both sides made significant accomplishments. The Trịnh created centralized government offices in charge of state budget and producing currency, unified the weight units into a decimal system, established printing shops to reduce the need to import printed materials from China, opened a military academy, and compiled history books.
Meanwhile, the Nguyễn Lords continued the southward expansion by the conquest of the remaining Cham land. Việt settlers also arrived in the sparsely populated area known as "Water Chenla", which was the lower Mekong Delta portion of Chenla (present-day Cambodia). Between the mid-17th century to mid-18th century, as Chenla was weakened by internal strife and Siamese invasions, the Nguyễn Lords used various means, political marriage, diplomatic pressure, political and military favors,... to gain the area around present day Saigon and the Mekong Delta. The Nguyễn army at times also clashed with the Siamese army to establish influence over Chenla.
In 1771, the Tây Sơn revolution broke out in Quynhơn, which was under the control of the Nguyễn. The leaders of this revolution were three brothers named Nguyễn Nhạc, Nguyễn Lữ, and Nguyễn Huệ, not related to the Nguyễn lords. By 1776, the Tây Sơn had occupied all of the Nguyễn Lord's land and killed almost the entire royal family. The surviving prince Nguyễn Phúc Ánh (often called Nguyễn Ánh) fled to Siam, and obtained military support from the Siamese king. Nguyễn Ánh came back with 50000 Siamese troops to regain power, but was defeated at the Battle of Rạch Gầm–Xoài Mút and almost killed. Nguyễn Ánh fled Vietnam, but he did not give up.
The Tây Sơn army commanded by Nguyễn Huệ marched north in 1786 to fight the Trịnh Lord, Trịnh Khải. The Trịnh army failed and Trịnh Khải committed suicide. The Tây Sơn army captured the capital in less than two months. The last Lê emperor, Lê Chiêu Thống, fled to China and petitioned the Chinese Qing Emperor for help. The Qing emperor Qianlong supplied Lê Chiêu Thống with a massive army of around 200,000 troops to regain his throne from the usurper. Nguyễn Huệ proclaimed himself Emperor Quang Trung and defeated the Qing troops with 100,000 men in a surprise 7 day campaign during the lunar new year (Tết). During his reign, Quang Trung envisioned many reforms but died by unknown reason on the way march south in 1792, at the age of 40.
During the reign of Emperor Quang Trung, Đại Việt was actually divided into 3 political entities. The Tây Sơn leader, Nguyễn Nhạc, ruled the centre of the country from his capital Qui Nhơn. Emperor Quang Trung ruled the north from the capital Phú Xuân Huế. In the South, Nguyễn Ánh, assisted by many talented recruits from the South, captured Gia Định (present day Saigon) in 1788 and established a strong base for his force.
After Quang Trung's death, the Tây Sơn Dynasty became unstable as the remaining brothers fought against each other and against the people who were loyal to Nguyễn Huệ's infant son. Nguyễn Ánh sailed north in 1799, capturing Tây Sơn's stronghold Qui Nhơn. In 1801, his force took Phú Xuân, the Tây Sơn capital. Nguyễn Ánh finally won the war in 1802, when he sieged Thăng Long (Hanoi) and executed Nguyễn Huệ's son, Nguyễn Quang Toản, along with many Tây Sơn generals and officials. Nguyễn Ánh ascended the throne and called himself Emperor Gia Long. Gia is for Gia Định, the old name of Saigon; Long is for Thăng Long, the old name of Hanoi. Hence Gia Long implied the unification of the country. The Nguyễn dynasty lasted until Bảo Đại's abdication in 1945. As China for centuries had referred to Đại Việt as Annam, Gia Long asked the Chinese Qing emperor to rename the country, from Annam to Nam Việt. To prevent any confusion of Gia Long's kingdom with Triệu Đà's ancient kingdom, the Chinese emperor reversed the order of the two words to Việt Nam. The name Vietnam is thus known to be used since Emperor Gia Long's reign. Recently historians have found that this name had existed in older books in which Vietnamese referred to their country as Vietnam.
The Period of Division with its many tragedies and dramatic historical developments inspired many poets and gave rise to some Vietnamese masterpieces in verse such as the epic poem The Tale of Kieu (Truyện Kiều) by Nguyễn Du, Song of a Soldier's Wife (Chinh Phụ Ngâm) by Đặng Trần Côn and Đoàn Thị Điểm, and a collection of satirical, erotically charged poems by the female poet Hồ Xuân Hương.