Even after the U.S. exited stage right from its infamous war in Southeast Asia, the region continued to experience dramatic political unrest. The divorce of Soviet and Sino relations had begun to affect the way that the People's Republic of China viewed its southern neighbours- particularly the Vietnamese, who had chosen to pursue an alliance with the USSR over China. The Vietnamese invasion into Cambodia in 1978, and toppling of the Sino aligned Khmer Rouge within, fanned the flames of China’s concerns about Soviet-backed expansionism. China launched an invasion in response, breaching the Vietnamese border on 17th of February in 1979.
The conflict continued over the course of nearly four weeks, after which both China and Vietnam declared the whole affair a victory, despite several factors that would make the opposite point. Gerald Segal, in his 1985 book Defending China, concluded that China’s 1979 war against Vietnam was a complete failure: “China failed to force a Vietnamese withdrawal from Cambodia, failed to end border clashes, failed to cast doubt on the strength of the Soviet power, failed to dispel the image of China as a paper tiger, and failed to draw the United States into an anti-Soviet coalition.”
Despite this, it could be argued that the Chinese enjoyed their own (albeit small) share of victories. The Soviet Union never offered Vietnam military assistance beyond sharing some intelligence, and that in and of itself proved that the Kremlin would choose not to wage war against the PRC, even to protect a close ally.
Even to this day, Sino-Vietnamese relations are uneasy. Their borders remain highly contested, and Vietnam maintains incredibly close diplomatic ties with the Russian federation.
A Vietnamese militia woman displaying Chinese men she captured as prisoners with their hands tied behind their backs, from the Chinese 41st and 42nd Army corps, Sino-Vietnamese War, 25 February 1979, Cao Bằng Province, Vietnam