There was once an uninhabited islet lying close to the Chinese side of the Ussuri River, which marks the border between Russia and China in the Far East. “Was,” because it has since begun to attach itself to the Chinese bank in a defiant act of geographic irony. But during the turbulent spring of 1969 this little islet — called Damansky in Russian and Zhenbao Dao in Chinese — was the stage for a game-changing encounter.

It was on this islet that on March 2 the Chinese set up an ambush, killing 31 Soviet border guards. The daring provocation was an effort to deter the Soviets from invading China, something that seemed only too possible after the Soviet invasion of Czechoslovakia in August 1968.

The fighting resumed two weeks later. The Soviets deployed tanks and bombarded the Chinese positions with BM-21 rockets, killing (in their estimate) up to a thousand Chinese troops. After several months of uneasy quiet, another skirmish broke out on Aug. 13, this time along the Western section of the border, in present-day Xinjiang. Twenty-one Chinese and two Soviets lost their lives.

The conflict was not entirely a surprise. Relations between the two Communist giants had been tense for a decade, with each accusing the other of betraying Marxism. The ideological quarrel obscured a more fundamental divergence: Mao Zedong was unwilling to subordinate himself to the Soviets in the rigid hierarchy of the Communist world. The Soviet leaders accused Mao of “great power chauvinism,” without recognizing that the label suited them equally well.