Cambodia–Laos border

The Mekong here is extremely wide, forming the Si Phan Don islands area, with the border along the southern rim of this riverine lake, before leaving it and proceeding overland to the north.

The border then turns east, reaching the Kong River and following this for a period, before proceeding eastwards over to the Vietnamese tripoint in the Annamite Mountains.

[2] Laos was at this point part of the Kingdom of Siam (the old name for Thailand), however all areas east of the Mekong were annexed to French Indochina in 1893 following the Franco-Siamese crisis.

[2][3][4] In 1905 the southern Lao regions of Stung Treng and Ratanakiri were transferred to Cambodia based on their historical affiliation to the Khmer kingdom, with a small area going to Vietnam.

[2][5] It is thought that the French authorities thereafter used the historic boundary of Stung Treng province as the Cambodia–Laos border, though as it was at this point merely an internal administrative line running through a remote area there was no pressing need to define it in detail.