Prior to russia’s invasion of Ukraine, Chinese President Xi Jinping was likely adding up the benefits of his warming relationship with Vladimir Putin. His Russian counterpart was pushing back against U.S. power, straining American alliances in Europe, and harassing a young democracy next door in Kyiv—all at almost no cost to China. Maybe, just maybe, Putin would even pave the way for Xi to achieve his paramount foreign-policy objective: claiming Taiwan.

Since the war began, however, the pitfalls of China’s partnership with Putin have revealed themselves all too clearly. A revitalized U.S. alliance network has collectively imposed damaging sanctions on Russia. Beijing has tried to do what it usually does—tap dance between all sides and pretend to be neutral—but finds itself an outlier among the world’s major powers. No one’s fooled about where Xi’s sympathies lie, and his stand is further alienating a strengthened transatlantic alliance.

These contradictions should come as no surprise. Ever since the Communist Party’s earliest days in China a century ago, its relations with Russia have held tremendous promise, but are too often plunged into peril. So they may be today.

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In recent decades, China’s position in the world was determined, to a great degree, by its relationship with the United States. But in many respects, Russia is as crucial to the story of modern China, for both good and bad reasons.

Relations between the two Communist regimes had an inauspicious beginning. In December 1949, just two months after he founded the People’s Republic following the Chinese civil war, Mao Zedong boarded an armored railway car to meet Joseph Stalin in Russia. Arriving hat in hand, the destitute supplicant before the undisputed don of the Communist world, Mao was the inheritor of a poverty-stricken, war-ravaged country desperate for money, technology, and international support. Russian help would be essential for his regime’s survival. When the two leaders met, Mao asked Stalin for just about everything: a treaty of alliance, financial aid, military assistance, even help editing his own writings. Stalin was encouraging but noncommittal, parking Mao in a dacha outside Moscow while hard bargaining dragged on for weeks. Mao eventually got his friendship pact, signed in February 1950, but on humiliating terms that evoked the hated “unequal treaties” imposed on China by imperial powers in the 19th century.

Still, the Russian came through with a gargantuan amount of aid, “the biggest such program undertaken by any country anywhere, including the U.S. Marshall Plan for Europe,” the historian Odd Arne Westad wrote in his book The Cold War: A World History. Soviet advisers trained Chinese army officers and helped plan Chinese cities. Moscow’s enthusiasm grew after Stalin’s death: The Soviet dictator’s successor, Nikita Khrushchev, believed that China was key to Communism’s ultimate victory over the West.

But relations began unraveling in the late 1950s. Mao came to resent his subordinate status in the Communist hierarchy and broke with Moscow on economic and foreign policy. To Mao, the Soviets suffered from “right-deviationist thinking”—fighting words in Communist lingo. In 1969, border skirmishes nearly escalated into full-on war. The Soviets threatened to use nuclear weapons, and Mao feared they just might. Tensions were defused through negotiations, but the intense rivalry between Beijing and Moscow propelled Mao to make a history-altering decision: to meet with President Richard Nixon in 1972 and reconcile with China’s supposed imperialist tormentor, the United States.