Chinese President Xi Jinping was the toast of Russia last week. He smiled at the Moscow Zoo as Russian President Vladimir Putin admired the pandas Xi had brought (a standard Chinese gift to countries it courts). In St. Petersburg, he toured the Aurora, the warship that fired the shot marking the start of the Bolshevik Revolution in 1917 and took an evening boat cruise with Putin. At the St. Petersburg Economic Forum, he quoted Fyodor Dostoevsky.

At a time when U.S. President Donald Trump — who once called his relationship with his Chinese counterpart "outstanding" — is waging a trade war against China, Xi needs a new "best friend." In his own words, that is what he has found in Putin. But is all this mutual affection really in Russia's best interest?

To be sure, this is not a new development. Over the last six years, Putin and Xi have met at least 30 times and annual trade between their countries amounts to more than $100 billion. But the bilateral relationship has deepened significantly lately, exemplified by last week's forum, which resulted in more than 25 trade and other agreements covering areas ranging from agriculture to technology. Both leaders gush that their two countries are now on better terms than ever.