Đàng Ngoài in the context of "Đàng Trong"

Vietnam, officially the Socialist Republic of Vietnam (SRV), is a country at the eastern edge of Mainland Southeast Asia. With an area of about 331,000 square kilometres (128,000 sq mi) and a population of over 100 million, it is the world's 15th-most populous country. One of two communist states in Southeast Asia, Vietnam is bordered by China to the north, Laos and Cambodia to the west, the Gulf of Thailand to the southwest, and the South China Sea to the east; it also shares maritime borders with Thailand, Malaysia, and Indonesia to the south and southwest, the Philippines to the east, and China to the northeast. Its capital is Hanoi, while its largest city is Ho Chi Minh City.

Vietnam was inhabited by the Paleolithic age, with states established in the first millennium BC on the Red River Delta in modern-day northern Vietnam. The Han dynasty annexed northern and central Vietnam, which were subsequently under Chinese rule from 111 BC until the first dynasty emerged in 939. Successive monarchical dynasties absorbed Chinese influences through Confucianism and Buddhism, and expanded southward to the Mekong Delta, conquering Champa. During most of the 17th and 18th centuries, Vietnam was effectively divided into two domains of Đàng Trong and Đàng Ngoài. The Nguyễn—the last imperial dynasty—surrendered to France in 1883. In 1887, its territory was integrated into French Indochina as three separate regions. In the immediate aftermath of World War II, the Viet Minh, a coalition front led by the communist revolutionary Ho Chi Minh, launched the August Revolution and declared Vietnam's independence from the Empire of Japan in 1945.

Haiphong or Hai Phong (Vietnamese: Hải Phòng, pronounced [hǎːj fɔ̀ŋ͡m] ) is the third-largest city in Vietnam and is the principal port city of the Red River Delta. The municipality has an area of 1,526.52 km (589.39 sq mi). The city includes Bạch Long Vĩ and Cát Hải islands in the Gulf of Tonkin. It has a population of 2,130,898 in 2023. The city's economy has strength in manufacturing, as evidenced by large industrial parks and numerous smaller traditional handicraft villages. Historically, Haiphong was the first place in Vietnam and Mainland Southeast Asia to get electricity.

In the imperial era of Đại Việt, the Bạch Đằng River in Haiphong was a place of many legendary victories, led by now-legendary commanders Ngô Quyền and Trần Hưng Đạo. In the 16th century, Mạc dynasty promoted the coastal settlement as a secondary capital, growing to become an important port town of Đàng Ngoài. After the French conquest of Vietnam, in 1888, the president of the French Third Republic, Sadi Carnot, promulgated a decree to establish Haiphong as one of the principal cities of French Indochina. From 1954 to 1975, Haiphong served as the most important maritime city of North Vietnam. It was one of the directly controlled municipalities of a reunified Vietnam with Hanoi and Ho Chi Minh City in 1976.

The Nguyễn lords (Vietnamese: Chúa Nguyễn, 主阮; 1558–1777, 1780–1802), also known as the Nguyễn clan (Vietnamese: Nguyễn thị; chữ Hán: 阮氏), were Nguyễn dynasty's forerunner and a feudal noble clan ruling southern Đại Việt in the Revival Lê dynasty. The Nguyễn lords were members of the House of Nguyễn Phúc. The territory they ruled was known contemporarily as Đàng Trong (Inner Realm) and known by Europeans as the Kingdom of Cochinchina and as Kingdom of Quảng Nam (Vietnamese: Quảng Nam Quốc; chữ Hán: 廣南國) by Imperial China, in opposition to the Trịnh lords ruling northern Đại Việt as Đàng Ngoài (Outer Realm), known as the "Kingdom of Tonkin" by Europeans and "Kingdom of Annam" (Vietnamese: An Nam Quốc; chữ Hán: 安南國) by Imperial China in bilateral diplomacy. They were officially entitled, in Sino-Vietnamese, the Nguyễn Vương (chữ Hán: 阮王) in 1744 when lord Nguyễn Phúc Khoát self-proclaimed himself to elevate his status equally to Trịnh lords's title known as the Trịnh Vương (Vietnamese: Trịnh Vương; chữ Hán: 鄭王). Both Nguyễn and Trịnh clans were de jure subordinates and fief of the Lê dynasty. However, the de jure submission of the Nguyễn lords to the Trịnh lords ended in 1627 sparked the war between them.

While they recognized the authority of and claimed to be loyal subjects of the revival Lê dynasty, they were de facto rulers of southern Đại Việt. Meanwhile, the Trịnh lords ruled northern Đại Việt in the name of the Lê emperor, who was in reality a puppet ruler. They fought a series of long and bitter wars that pitted the two halves of Vietnam against each other. The Nguyễn were finally overthrown in the Tây Sơn wars, but one of their descendants would eventually come to unite all of Vietnam. Their rule consolidated earlier southward expansion into Champa and pushed southwest into Cambodia.

Tonkin, also spelled Tongkin, Tonquin or Tongking, is an exonym referring to the area surrounding the Red River Delta of Northern Vietnam. During the 17th and 18th centuries, this term referred to the domain of Đàng Ngoài, under the control of the Trịnh lords, encompassing both the Northern and Thanh–Nghệ regions, north of the Gianh River at the 17.7th parallel. From 1884 to early 1945, the term referred to the French protectorate of Tonkin, which comprised only the Northern region, approximately north of the 20th parallel.

The Trịnh lords (Vietnamese: Chúa Trịnh; Chữ Hán: 主鄭; 1545–1787), formally titled as Viceroy of Trịnh (Vietnamese: Trịnh Vương ; chữ Hán: 鄭王), also known as the House of Trịnh or the Trịnh clan (Trịnh thị; 鄭氏), were a feudal noble clan that ruled Đàng Ngoài (known by the exonym Tonkin) during the Revival Lê dynasty. The Trịnh lords were de jure subordinates of the Lê dynasty emperors but were in actuality the de facto rulers of northern Vietnam.

The Trịnh clan and their rivals, the Nguyễn clan, were called "Chúa" (主, Lord) by their subjects and controlled northern and southern Vietnam respectively, leaving the Later Lê emperors as rulers in name only. The Trịnh clan produced 12 lords who dominated the royal court of the Later Lê dynasty and ruled northern Vietnam for more than two centuries.

The Trịnh–Nguyễn Civil War (Vietnamese: Trịnh-Nguyễn phân tranh; chữ Hán: 鄭阮紛爭, lit. Trịnh–Nguyễn contention) was a lengthy civil war during the 17th-18th centuries waged between the two ruling families in Vietnam, the Trịnh lords of Đàng Ngoài and the Nguyễn lords of Đàng Trong, centered in today's Central Vietnam. The wars resulted in a long stalemate and century of peace before conflicts resumed in 1774 resulting in the emergence of the Tây Sơn forces following Trịnh Lords at the time, Nguyễn Lords collapsed in Saigon in 1777. During the division of Vietnam, Gianh River was used as the de facto border between both sides in peacetime.