In Beijing’s view, in the absence of an explicit treaty provision, the central line of the main channel—the Thalweg principle—provided a legal basis for delimiting the boundary in the two rivers. On this basis, Beijing claimed that 600 of the rivers’ 700 islands—including Zhenbao Island on the Ussuri River, just 180 miles southwest of an important Soviet city, Khabarovsk—belonged to the P.R.C.
Later, in the 1980s, western scholars found that, notwithstanding the substantial Soviet military buildup near the Chinese borders in the late 1960s, it was Beijing that initiated the clash on Zhenbao (Damanski) Island. Peter Zwick has argued that the Soviets were in a generally reactive mode during the height of the conflict and exhibited restraint in responding to what they saw as Chinese provocations.
In mid-1969, both countries fought a brief border war over Zhenbao Island[13], along the Ussuri River halfway between Vladivostok and Khabarovsk, killing dozens of conscripts on either side.
2016 September 29, “Scenery of Zhenbao Island wetland in Heilongjiang(1/6)”, in Li Yan, editor, China News Service, archived from the original on 6 October 2016:
Photo taken on Sept. 28, 2016 shows the scenery of the Zhenbao Island wetland in Hulin, northeast China's Heilongjiang Province. The 29,275-hectare wetland has been designated as a wetland of international importance by the Ramsar Convention in 2011 for its vital importance as a unique wetland for biological diversity.
2021 March 29, Blake Herzinger, “China’s Military Could Turn Small Clashes Into Major Conflicts”, in Foreign Policy, archived from the original on 29 March 2021:
In 1969, Chinese forces instigated a brutal ambush on unsuspecting Soviet border guards at Zhenbao Island, a contested bank on the Ussuri River. After numerous rough but not lethal hand-to-hand scuffles in the past, the island became the epicenter of a pitched battle and weeks-long exchange of fire. The initial ambush, conducted with small arms, escalated into deployment of heavy armor supported by rocket artillery. Hundreds of people were killed. Within two weeks, both sides were contemplating the possibility of a nuclear exchange.