The aggressive Chinese Communists added Vietnam to their list of victims when it invaded on February 17, 1979 with 120,000 troops. This was in reaction to Vietnam invading Cambodia and liberating it from the terror of the Khmer Rouge whom the Chinese government supported.
The Vietnamese were no match for their giant, aggressive neighbor as they easily marched into Vietnamese territory and blocked Vietnamese counterattacks into Yunnan Province. Within a month, they left Vietnam, satisfied that they had been taught a lesson.
The Vietnamese, no doubt had territorial ambitions in Cambodia, but China's support for the brutal Khmer regime (which killed up to one-third of Cambodia's entire population) and then their invasion of their much smaller neighbor who ended the Cambodian genocide is par for the course for the Chinese. As they get more powerful, will (and already have) continue to push their weight around the region in the name of tyranny.
OnWar.com
Federation of American Scientists
A Nervous China Invades Vietnam