It has been over 40 years since Vietnamese military forces rolled into Cambodia and toppled the brutal Pol Pot regime. One of the drivers that led Vietnam to conduct a military campaign against Cambodia was its sense of strategic vulnerability, sandwiched between China to the north and the China-backed Cambodian regime to the south.
A declassified CIA document from February 1983 suggests that China encouraged the Pol Pot regime ‘in its hostility toward Vietnam’ in the years leading up to the invasion. During this time, the Khmer Rouge carried out raids in southern Vietnam while receiving weapons from China, including T-54 and T-63 tanks. Pol Pot’s refusal to engage diplomatically with Vietnam to settle their border disputes prompted the Vietnamese to launch a large-scale invasion of Cambodia in 1978.
Today, Vietnam is once again concerned about China’s growing defence ties with Cambodia. Hanoi fears that China is using Cambodia as a tool to pressure Vietnam on its southern border.
In full: https://www.eastasiaforum.org/2023/05/03/chinas-defence-ties-with-cambodia-raise-red-flags-for-vietnam/