Vietnam's decision to invade Cambodia in December 1978 was driven by various factors. Firstly, there were security concerns as the Khmer Rouge launched cross-border attacks on Vietnamese border villages, resulting in civilian casualties. Secondly, Vietnam sought to remove the Khmer Rouge from power due to ideological differences and the inhumanity committed by the regime. Additionally, Vietnam aimed to install a pro-Vietnamese government in Cambodia to protect its strategic interests and counter the influence of China, which supported the Khmer Rouge.