IIRC the Chinese were stealing a lot of Soviet-made aid meant for Vietnam and supplying the Vietnamese with Chinese-made knock-offs instead. The Soviets eventually said "fuck this" and started sending stuff to Vietnam by sea because the US Navy wasn't going to intercept and pillage the shipments.
EDIT: That war was the Chinese response to the Vietnamese Communists saying "enough is enough" and then getting rid of Pol Pot while restoring the Cambodian Monarchy.