As a little kid, I vaguely remember seeing on the news reports about the Khmer Rouge taking control of Phnom Penh and, later, Vietnam invading the country. I remember someone half-joking that even the Vietnamese Communists thought the Cambodian Communists were too crazy and intervened to straighten them out.
Vietnam was more in the Soviet camp, whereas the Khmer Rouge was supported by China.
Even Ho Chi Minh remarked that he would rather smell French s___ for five years, than Chinese s___ for a thousand years.
And also in 1979, there was a brief border skirmish between Vietnam and China.