China–Vietnam border

The border region has a long history. China under the Han dynasty had established control over Nam Việt (northern Vietnam) by the early 1st century BC.[4][5] The Vietnamese of Tonkin managed to break free of Han rule in 939 AD, forming the kingdom of Đại Việt.[6][4] An initial boundary between the two kingdoms at roughly its present location was established in the aftermath of the Lý–Song War (1075-1077).[7] Thereafter relations were largely peaceful, save for a 20 year occupation of northern Vietnam by Ming China in the 15th century.[4][8][9]

Pillars marking the border near Bản Giốc

In the 19th century France began taking an interest in Vietnam, conquering Cochininach by 1862.[4] It then expanded its rule over Annam (central Vietnam) and then Tonkin (northern Vietnam) in 1883-84, alarming China and precipitating the Sino-French War.[10] Hostilities were ended via the Treaty of Tientsin (1885), which provided for the demarcation of the border between China and Tonkin. Article 3 of the treaty provided for the appointment of a Sino-French commission to demarcate the border between Tonkin and China, which forms today's China–Vietnam border. China's commissioners were Chou Te-jun (周德潤), Hung Lu-ssu (鴻臚寺) and Ch'ing Teng-ch'eng (卿鄧承). The French commission was led by M. Bourcier Saint-Chaffray, and its members included M. Scherzer, the French consul in Canton, Dr Paul Neis, a noted Indochina explorer, Lieutenant-Colonel Tisseyre, Captain Bouinais, and M. Pallu de la Barrière (though the latter took no part in the commission's work). In preparation for the commission's work General de Courcy despatched French troops to occupy Lạng Sơn, That Khe and other border towns in October 1885.

Demarcation work began in late 1885 and was completed in 1887, the latter the same year that Tonkin was incorporated into the colony of French Indochina.[4] The French rejected Chinese claims to the Vietnamese town of Đồng Đăng, close to the Guangxi border and the site of a French victory during the Sino-French War, but agreed that the Pak-lung peninsula (白龍尾) on the western border of Guangdong province should be awarded to China. A dispute over two areas on the border between Yunnan province and Tonkin was settled by the award of Meng-suo (猛梭) and Meng-lai (猛賴) to Vietnam and the transfer of a large tract of fertile arable land between Ma-pai-kuan (馬白關) and Nan-tan-shan (南丹山) to China. An agreement confirming the new border between Vietnam and China was signed in Peking on 26 June 1887 by French and Chinese representatives.[11][4] A later convention, signed on 20 June 1895, delimited some disputed areas of the frontier.[4] Subsequently 285-341 border markers[lower-alpha 1] were erected along the boundary.[2] The border demarcation between the French and the Qing Dynasty was described as being carried out under "incomplete technical and practical conditions and incomplete and unclear and inaccurate texts and maps of many border sections." The border markers were erected not based on a coordinate grid and many suffered damage, were lost through time or removed. Many of the original maps also no longer existed and migration of populations took place that was not in accordance with the de jure borderline. All these factors posed difficulties to border management.[2]

French Indochina was invaded by Japan via China during the Second World War, though a compromise was arranged whereby a Vichy French administration continued to run the colony whilst Japanese troops were allowed to be stationed there.[12][13] This modus vivendi broke down in the final year of the war - a short-lived Vietnamese puppet-state was established by the Japanese, which later collapsed, prompting the nationalist Việt Minh movement under Hồ Chí Minh to declare independence.[14] A long war then broke out as France attempted to reassert control, ending in their defeat and the independence of Vietnam; however the country was split in two, with North Vietnam like China now under Communist rule.[15] In 1957 and 1958 China and North Vietnam confirmed that they both respected the border established via the Tientsin Protocol.[16][17]

During the Vietnam War China backed North Vietnam with arms and aid, assisting the North in its eventual victory.[18][19] However after the war ended in 1975 relations deteriorated over several contentious issues, such as Vietnam's siding with the USSR in the Sino-Soviet split, China's rapprochement with the USA under President Richard Nixon, disputes over the maritime border, and China's alleged support for minority insurgent groups along the border.[16][20][21] The final straw came after Vietnam invaded Cambodia and removed the Chinese-backed Khmer Rouge government in 1978.[22] The following year the brief Sino-Vietnamese War broke out along the border; though China withdrew after only three weeks it remained in occupation of small strategic areas along the frontier, and low-level skirmishes along the border continued throughout the 1980s.[21][23][24]

In 1990 Vietnam withdrew from Cambodia, ending one of the major points of contention between China and Vietnam, and relations were normalised in November 1991.[25] In October 1992 the two countries began work on resolving outstanding territorial issues.[16] Both sides accepted the validity of the 1887 and 1895 lines, and a final boundary treaty was then signed on 30 December 1999.[16][17][21] The treaty was ratified the following year, and full demarcation then began, which was completed in 2008.[25][17] Out of a disputed territory of 227 km² (87.6 mi²), China acquired 114 km² (44 mi²) and Vietnam acquired 113 km² (43.6 mi²).[26] In 2000 an agreement on the maritime boundary in the Gulf of Tonkin was agreed, however disputes persist over islands in the South China Sea.[25]