Can the trans-Atlantic relationship be saved? That’s the question the world faces 100 years after the end of World War I.

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,...