In a massive campaign to destroy supposed enemies of communism the Khmer Rouge

Vietnam launched an invasion of Cambodia in late December 1978 to remove Pol Pot. Two million Cambodians had died at the hands of his Khmer Rouge regime and Pol Pot's troops had conducted bloody cross-border raids into Vietnam, Cambodia's historic enemy, massacring civilians and torching villages

Explanation:

(202 bc - 220 ad) dynasty started by lui bang; a great and long-lasting rule, it discarded the harsh policies of the qin dynasty and adopted confucian principles; han rulers chose officials who passed the civil service exams rather than birth; it was a time of prosperity