Complex sentence: In response, the PRC denounced the Vietnamese deposition of their Maoist client-leader, and retaliated by invading northern Vietnam, in the Sino-Vietnamese War (1979); in turn, the USSR denounced the PRC's invasion of Vietnam.
Split into simpler sentences: In response, the PRC denounced the Vietnamese deposition of their Maoist client-leader, and retaliated by invading northern Vietnam, in the Sino-Vietnamese War (1979). In their turn, the USSR denounced the PRC's invasion of Vietnam.