At first I had no idea why President Trump talked so much about sovereignty. I’ve changed my mind. To be more precise, Xi Jinping changed it. Mr. Trump is the only thing that stands between us and a world dominated by China.

“We do not expect diverse countries to share the same cultures, traditions, or even systems of government,” Mr. Trump told the United Nations General Assembly in September 2017. “But we do expect all nations to uphold these two core sovereign duties: to respect the interests of their own people and the...