In 1969, China and the Soviet Union seemed on the brink of war.

They fought a deadly border clash in March of that year and another in August. The Kremlin dropped hints of a nuclear strike. Over the next few years, they exchanged barbs. Mao Zedong warned, “You piss on my head, and I shall retaliate!” The Soviet leader Leonid Brezhnev called Mao “treacherous.” An alliance that Moscow and Beijing previously billed as unbreakable quickly unraveled.

So Mao reached out to his avowed foe the United States. Mao, a scathing critic of what he called American imperialism, suddenly referred to President Richard Nixon as “the No. 1 good fellow in the world,” and by 1972, Nixon turned up in Beijing. It was a geopolitical earthquake that altered the course of history.

These days Vladimir Putin is Xi Jinping’s No. 1 good fellow as the two countries make common cause against the United States. But the Russian leader — his authority bruised in the wake of the aborted mutiny by the Wagner paramilitary group in June — would be wise to keep in mind China’s track record. As Mikhail Kapitsa, a top Soviet foreign ministry official, put it in 1982, “The Chinese never befriend anyone for a long time.”

The Chinese Communist Party’s approach to geopolitics is rooted in an ancient strategic culture of playing other nations — sometimes dismissed as barbarians during China’s imperial times — against one another for China’s benefit. Mao’s abrupt turn to the United States showed just how quickly Chinese loyalties can crumble when the usefulness of a strategic partner wanes. . . .

Mr. Xi, perhaps sensing diminishing returns from deeper engagement with America, has brought things full circle once more during the Putin era, embracing the Russian leader and denouncing the United States.

The West is right to be worried. Turning the clock back to the days of Sino-Soviet brotherhood, Mr. Putin and Mr. Xi have unequivocally sided with each other in challenging the Western-led world order. The combination of Mr. Putin’s revanchism and military aggression and China’s economic power is dangerous.

But Mr. Putin has made a potentially grave error, burning bridges with the West to go all in with China in reckless disregard for Beijing’s track record of instrumentalizing its friendships.

Despite offering diplomatic cover for Mr. Putin’s invasion of Ukraine, China has largely avoided running afoul of Western sanctions on Russia, essentially putting its own interests before those of its embattled client. Russia’s deepening isolation has given China access to discounted Russian energy products. Much of the trade between China and Russia is now conducted in the Chinese yuan, which reduces Russia’s exposure to Western economic pressure but also advances Beijing’s goal of undercutting the dollar’s dominance as the world currency. At the same time, China has been able to portray itself to much of the world as a responsible global player with its halfhearted calls for peace in Ukraine as the war has dragged on.

Mr. Putin, on the other hand, has made his country a junior partnerto China. Looking weakened and less secure after the Wagner revolt last month, he risks becoming even more dependent on China for political and economic support.

Mr. Xi will no doubt take note. Like past Chinese leaders, he respects strength but knows how to exploit weakness, and Russia will remain useful to him as he continues to challenge the United States. Mr. Putin can still make major strategic choices for his country, as long as they coincide with China’s interests. But will China stand by him if those interests diverge? Or if Russian elites run out of patience with his poor decisions and try to push him out? Or if the global costs of standing with him prove too onerous for China?

China remains the same secretive, self-serving Communist Party state that it was in Mao’s day, with an outlook on global politics in which alignments are viewed as temporary. There are no “good feelings,” as Mr. Geng put it five decades ago, just cold calculation.

The West, so concerned today about this newly united front between China and Russia, should remember that.

So should Mr. Putin.