The U.S. is running out of time to prevent a cataclysmic war in the Western Pacific. While the world has been focused on Vladimir Putin’s aggression in Ukraine, Xi Jinping appears to be preparing for an even more consequential onslaught against Taiwan. Mr. Xi’s China is fueled by a dangerous mix of strength and weakness: Faced with profound economic, demographic and strategic problems, it will be tempted to use its burgeoning military power to transform the existing order while it still has the opportunity.This peaking-power syndrome—the tendency for rising states to become more aggressive as they become more fearful of...