China and Russia completed an eight-day joint naval exercise in the South China Sea last week, and this time the location was also the message. The two autocracies are expanding cooperation and offering each other support in their territorial disputes, a trend that could fuel instability from East Asia to Central Europe.

Days before the drill, which focused on antisubmarine warfare and what a Xinhua dispatch called “island-seizing,” Vladimir Putin and Xi Jinping held their 15th bilateral meeting on the sidelines of the G-20 summit in Hangzhou. Mr. Putin announced support for Beijing’s aggressive sovereignty claims in the South China Sea and opposition to “any third-party interference,” an unsubtle reference to the United States.

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