RUTHFULLY YOURS

The signs from the centennial commemorations in Paris were not good. French President Emmanuel Macron publicly condemned nationalism as “the opposite of patriotism” as self-proclaimed nationalist Donald Trump looked on stonily. The relationship between the U.S. and its three principal European allies—Germany, Britain and France—is arguably cooler than at any time since the Truman administration.

Paradoxically, the chill has occurred just as the security, economic and even ideological interests of the leading Western states have grown increasingly aligned. Russia and China both seek a weaker European Union, a divided Western alliance, and a decline in American power. China’s aggressively mercantilist economic plans target the capital-goods and automotive industries at the core of the German economy. In a world with better leadership, the major European states and the U.S. would deepen their partnership to prepare for a challenging new era in world politics. In our world, however, bitterness and resentment fester on both sides of the ocean, and the alliance weakens as the need for it grows.

The oracles of conventional wisdom naturally blame Mr. Trump—and they’re not all wrong. His negotiating style with Germany and France has been abrasive. From Iran to trade to the Intermediate-Range Nuclear Forces Treaty to the environment, he assaults what most Europeans see as their interests even as his “America First” rhetoric grates on their sensibilities.

But if Mr. Trump is wrong about many things, on one big issue he is right. However tangled its history, nationalism is an important force in global affairs that world leaders should respect. Mr. Macron’s disdainful remarks made for good headlines, but his inability to appreciate the role of nationalism in world politics exemplifies the failure of imagination at the root of many of Europe’s troubles.

The instinctive antinationalism of leaders like Mr. Macron is rooted in the belief that Western Europe is the real Europe and that its history is a universal history with lessons equally compelling for the rest of the world. These egotistical beliefs are so deeply held among elites in Western Europe that they are often unconscious.

In fact, the lessons of World War I were not the same everywhere. In Eastern and Central Europe, the war demonstrated the value, not the dangers, of nationalism. It broke the transnational bureaucratic empires that denied Poles, Lithuanians, Czechs and many others their freedom.

After World War II, the subjugation of these peoples in another multiethnic, bureaucratic imperial system, the Soviet bloc, confirmed their belief that the cause of nationalism was the cause of freedom. The Eastern and Central European countries that joined the EU did so because they believed membership in a prosperous Western club would, along with the North Atlantic Treaty Organization, preserve their national independence. They were escaping the ideological transnationalism of the Warsaw Pact, not signing up for another round of rule by “postnational” bureaucrats.

In the U.S., World War I never had the social and cultural impact that it did in Germany and France. Both the Civil War (in which as many as 750,000 Americans were killed) and World War II (418,500) loom much larger in American historical memory than World War I (116,500). The lesson of the 20th century for many Americans was that nationalism is a good thing. Nationalism unified the ethnically and religiously diverse American people at home and enabled the U.S. to triumph in two world wars and the Cold War.

Postnationalism is a Western fantasy, not a global trend, and no lasting peace can be built on such a shaky foundation. China, Russia, India, Pakistan, Vietnam, Brazil, Turkey—these countries and many others are as nationalistic as France was in 1910, and their nationalism is shaping a new reality in world affairs that puts both European and American security increasingly at risk.

Mr. Trump instinctively understands this, and he knows Western countries need the strong whiskey of nationalism—not just the weak tea of bureaucratic cosmopolitanism—to thrive in a dangerous world. That insight is not enough to anchor the West or ground U.S. strategy, but without it there is no foundation for security or peace.

If the American and European establishments had understood the importance of nationalism to the coherence of their own societies as well as the architecture of world order, Mr. Trump might still be producing television shows, and nobody would have heard of the Alternative for Deutschland. The longer mainstream leaders remain blind to nationalism’s importance, the more chaotic the world is likely to become.