What would become of the world if the United States became a normal great power? This isn’t to ask what would happen if the United States retreated into outright isolationism. It’s simply to ask what would happen if the country behaved in the same narrowly self-interested, frequently exploitive way as many great powers throughout history—if it rejected the idea that it has a special responsibility to shape a liberal order that benefits the wider world. That would be an epic departure from 80 years of American strategy. But it’s not an outlandish prospect anymore.

In 2016, Donald Trump won the presidency on an “America first” platform. He sought a United States that would be mighty but aloof, one that would maximize its advantages while minimizing its entanglements. Indeed, the defining feature of Trump’s worldview is his belief that the United States has no obligation to pursue anything larger than its own self-interest, narrowly construed. Today, Trump is again vying for the presidency, as his legion of foreign policy followers within the Republican Party grows. Meanwhile, fatigue with key aspects of American globalism has become a bipartisan affair. Sooner or later, under Trump or another president, the world could face a superpower that consistently puts “America first.”

That version of the United States wouldn’t be a global dropout. On some issues, it might be more aggressive than before. But it would also be far less concerned with defending global norms, providing public goods, and protecting distant allies. Its foreign policy would become less principled, more zero-sum. Most broadly, this version of the United States would wield outsized power absent any outsized ethos of responsibility—so it would decline to bear unequal burdens in pursuit of the real but diffuse benefits the liberal order provides.

The results would not be pretty. A more normal U.S. foreign policy would produce a world that would also be more normal—that is, more vicious and chaotic. An “America first” world could be fatal for Ukraine and other states vulnerable to autocratic aggression. It would release the disorder U.S. hegemony has long contained.

Yet the United States itself might not do so badly—at least for a while—in a world where raw power matters more because the liberal order has been gutted. And even if things really fell apart, Americans would be the last ones to notice. “America first” is so seductive because it reflects a basic truth. The United States would ultimately suffer in a more anarchic world—but between now and then, everyone else would pay the greater price.

All countries pursue their interests, but not all countries define those interests the same way. The concept of national interest traditionally emphasized the protection of one’s territory, population, wealth, and influence. Since World War II, however, most American leaders and elites have rejected the notion that it should be a normal country acting in normal way. After all, the war had demonstrated how the normal rhythms of international affairs could plunge humanity, and even a distant United States, into horror. It had thereby discredited the original “America First” movement, made up of opponents of U.S. intervention in World War II—and made clear that the world’s mightiest country must radically enlarge its view of what its interests entailed.

The resulting project was unprecedented in scope. It involved forging alliances that circled the globe and protected countries thousands of miles away, rebuilding devastated countries and creating a thriving free world economy, and cultivating democracy in distant lands. Not least, it meant abjuring the policies of conquest and naked exploitation that other great powers had so commonly pursued, and instead defending norms—nonaggression, self-determination, freedom of the commons—that would offer humanity a more peaceful and cooperative path. The United States was now assuming “the responsibility which God Almighty intended,” President Harry Truman declared in 1949, “for the welfare of the world in generations to come.”

This language of “responsibility” was revealing. American policymakers never doubted that their country would benefit from living in a healthier world. But creating that world required Washington to calculate issues of self-interest in a remarkably capacious way. No prior definition of national interest had required the world’s most secure, invulnerable country to risk nuclear war over territories on distant continents, or to rebuild former enemies as industrial dynamos and economic competitors. And no prior definition of national interest required making dramatically unequal contributions to the common security so one’s allies could deliberately underspend on their own defense.

“I see the advantages to the Western world,” President John Kennedy griped, in the early 1960s, of one such arrangement—Washington’s role in stabilizing and lubricating the international economy. “But what is the national, narrow advantage” for the United States? U.S. policy only made sense if one believed that the pursuit of national, narrow advantage had previously consigned the world to carnage—so Washington must create a larger international climate that benefited Americans by benefiting like-minded peoples around the globe. “The pattern of leadership,” Secretary of State Dean Acheson had explained in 1952, “is a pattern of responsibility.” Americans must “take no narrow view of our interests but . . . conceive of them in a broad and understanding way.”

One doesn’t have to think that everything has been wonderful since 1945 to recognize that history changed fundamentally once this “pattern of........

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