In an unusual diplomatic move, Chinese leader Xi Jinping initiated a phone call with President Trump on Monday to discuss Taiwan, a flashpoint that has surged to the forefront in recent days as Japan takes a more assertive stance on the island’s autonomy.

With Japan’s new leader refusing to back down from China’s show of force and claims on Taiwan, Xi Jinping picks up the phone to try to pry the U.S.-Japan alliance apart.

Takaichi's response to a question in parliament earlier this month that a hypothetical Chinese attack on Taiwan could trigger a Japanese military response prompted a furious response from Beijing that...