Although there was limited minelaying between the late 1960s and mid 1970s, the first significant use of landmines did not occur in Cambodia until the 1979 – 1989 Vietnamese occupation.
In December 1978 Vietnam intervened and the Khmer Rouge retreated to, and fought to defend, base camps along the north-west border. Then, through a series of dry season offensives in 1984-1985, the Vietnamese military drove the Khmer Rouge (and 230,000 civilians) across the border into Thailand. To impede the return of the Khmer Rouge tens of thousands of local people were forcibly conscripted over 18 months into assisting in the construction of a barrier minefield along the entire 750 kilometer length of the Cambodia-Thai border.
This fifth in a series of defensive plans (Kar Korpier pram) has become known infamously as “the K5”. During the decade that culminated in the final collapse of the remaining Khmer Rouge leadership (Anlong Veaeng, December 1998) further landmines were laid by State of Cambodia forces to defend towns and villages, military positions and supply routes from attack by opposition forces. In the same period Khmer Rouge and Monarchist opposition forces used landmines to protect newly won ground or to contaminate the interior of abandoned Vietnamese defensive positions.