Vladimir Putin is still useful to Xi Jinping. Until he isn't.

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 “You piss on my head, and I shall retaliate!" Soviet leader Leonid Brezhnev called Mao "treacherous." An alliance that Moscow and Beijing previously billed as unbreakable quickly unravelled. 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…