<P> The Mekong has thus not been an obstacle but a facilitator for communication, and the similarities between Laos and northeast Thai society--same people, almost same language--reflect the close contact that has existed across the river for centuries . Also, many Laotians living in the Mekong Valley have relatives and friends in Thailand . </P> <P> Prior to the twentieth century, Laotian kingdoms and principalities encompassed areas on both sides of the Mekong, and Thai control in the late nineteenth century extended to the left bank . Although the Mekong was established as a border by French colonial forces, travel from one side to the other has been significantly limited only since the establishment of the Lao People's Democratic Republic (LPDR, or Laos) in 1975 . </P> <P> The eastern border with Vietnam extends for 2,130 kilometres, mostly along the crest of the Annamite Chain, and serves as a physical barrier between the Chinese - influenced culture of Vietnam and the Indianized states of Laos and Thailand . These mountains are sparsely populated by tribal minorities who traditionally have not acknowledged the border with Vietnam any more than lowland Lao have been constrained by the 1,754 - kilometre Mekong River border with Thailand . Thus, ethnic minority populations are found on both the Laotian and Vietnamese sides of the frontier . Because of their relative isolation, contact between these groups and lowland Lao has been mostly confined to trading . </P> <P> Laos shares its short--only 541 kilometres--southern border with Cambodia, and ancient Khmer ruins at Wat Pho and other southern locations attest to the long history of contact between the Lao and the Khmer . In the north, the country is bounded by a mountainous 423 - kilometre border with China and shares the 235 - kilometre - long Mekong River border with Burma . </P>

What natural feature forms most of the border between laos and thailand