On Sept. 21, Chinese President Xi Jinping convened an emergency meeting of two dozen top officials. The day before, the U.S. had taken Beijing by surprise by imposing sanctions on a research unit of the Chinese military, shortly after announcing tariffs on $200 billion in Chinese imports. The Chinese didn’t know how to respond.

Mr. Xi arranged the meeting so hastily that three of the seven members of the group’s Standing Committee—China’s final arbiter of power—couldn’t attend because they were traveling, say individuals with...