Europe is at a crossroads. The high-water mark of trans-Atlantic security cooperation is behind us, and the Trump administration regards most of Europe with a combination of contempt, disdain, or outright hostility. At a minimum, Europe’s leaders can no longer take U.S. support and protection for granted. They can hope for the best, but they must plan for the worst. And that means charting an independent course in world politics.

To be fair, this situation isn’t entirely President Donald Trump’s fault. Even if he had never been elected, a fundamental rebalancing of trans-Atlantic relations was long overdue. A glance at a globe tells you why: The United States is not a European power, and a permanent U.S. military commitment there is a historical and geopolitical anomaly. A costly commitment of this sort can only be justified by clear strategic necessity, such as the desire to prevent any single power from dominating the entire region. This strategic objective is the main reason the United States entered World War I and World War II and why it kept substantial forces in Europe during the Cold War.

Europe is at a crossroads. The high-water mark of trans-Atlantic security cooperation is behind us, and the Trump administration regards most of Europe with a combination of contempt, disdain, or outright hostility. At a minimum, Europe’s leaders can no longer take U.S. support and protection for granted. They can hope for the best, but they must plan for the worst. And that means charting an independent course in world politics.

To be fair, this situation isn’t entirely President Donald Trump’s fault. Even if he had never been elected, a fundamental rebalancing of trans-Atlantic relations was long overdue. A glance at a globe tells you why: The United States is not a European power, and a permanent U.S. military commitment there is a historical and geopolitical anomaly. A costly commitment of this sort can only be justified by clear strategic necessity, such as the desire to prevent any single power from dominating the entire region. This strategic objective is the main reason the United States entered World War I and World War II and why it kept substantial forces in Europe during the Cold War.

These policies made good sense back then, but the Cold War has been over for more than 30 years, and the unipolar moment ended some years ago, too. China is now America’s main great-power rival and a potential regional hegemon, and the United States needs to focus its finite resources and energies on preventing Chinese hegemony in Asia. The good news is that no country is powerful enough to dominate Europe today—not even Russia—which means the........

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