Historically the
Annamite Range formed a natural boundary between Vietnamese kingdoms in the east and Lao, Thai and Khmer kingdoms in the west. Laos was at this point part of the
Kingdom of Siam (the old name for Thailand), however it was
annexed to French Indochina in 1893 following the
Franco-Siamese crisis. The precise date of the delimitation of the frontier is unclear; the International Boundary Study states that
"The juridical basis of the Laos–Viet-Nam boundary probably stems from ancient treaties and custom as modified or made more specific by decrees of the Indochinese administration." Part of the border was demarcated in 1916 following a dispute, and French maps were drawn up during the colonial period that were used as the basis for the later international border. Laos obtained a partial independence from France in 1949, with Cambodia gaining complete independence in 1953, followed by Vietnam in 1954. Vietnam was however partitioned into
North and
South Vietnam separated by a
Vietnamese Demilitarized Zone, with Laos bordering both entities. The
Kampuchean People's Revolutionary Party (KPRP) was founded on 28 June 1951 before it came to power in January 1979. During the
Vietnam War the border was crossed by
Ho Chi Minh's
Viet Cong supply lines along with the
communist movements such as
Pathet Lao led by
Souphanouvong and
Kaysone Phomvihane, the
Khmer Rouge led by Angkar ("Organization") and
Saloth Sâr ("Pol Pot"), most notably the
Ho Chi Minh Trail, causing it to be heavily bombed by American forces. Following the victory of the Communists in 1975 in Vietnam, Cambodia and Laos, a border treaty was signed in 1976 based on the colonial-era border line, which was formed as "Communist Indochina" or "Indochinese Federation" after the fall of
Phnom Penh,
Saigon and
Vientiane as the war had ended, with a new communist government organizations coming to power such as the
Communist Party of Vietnam (CPV), the
Communist Party of Kampuchea (CPK) and
Lao People's Revolutionary Party (LPRP) as a new system for the new society as a whole, which then formed with the constitutions as it was renamed the
Socialist Republic of Vietnam,
Democratic Kampuchea and
Lao People's Democratic Republic both declared by the party. But for Cambodia after it was declared
Year Zero, Pol Pot's reign of terror became responsible for the deaths of 1.5 up to nearly 2 million people, about a quarter of the whole population before it was ousted by Vietnam during the
Cambodian–Vietnamese War to end the infamous
genocide that almost eradicating everything by the cruelest and vicious Khmer Rouge cadres which then forced to flee back into the
jungle near the
Thai border after taking the capital
Phnom Penh by the
Vietnamese Army on 7 January 1979 led by its leader
Lê Duẩn. On the ground demarcation then took place from 1979 to 1984. Some small modification of the border were made in 1986. ==Border crossings==