Stephen M. Walt is the Robert and Renee Belfer Professor of International Affairs at Harvard University’s Kennedy School of Government.

The United States has been the dominant world power since 1945, and U.S. leaders have long sought to preserve that privileged position. They understood, as did most Americans, that primacy brought important benefits. It made other states less likely to threaten America or its vital interests directly. By dampening great-power competition and giving Washington the capacity to shape regional balances of power, primacy contributed to a more tranquil international environment. That tranquility fostered global prosperity; investors and traders operate with greater confidence when there is less danger of war. Primacy also gave the United States the ability to work for positive ends: promoting human rights and slowing the spread of weapons of mass destruction. It may be lonely at the top, but Americans have found the view compelling.

When a state stands alone at the pinnacle of power, however, there is nowhere to go but down. And so Americans have repeatedly worried about the possibility of decline—even when the prospect was remote. Back in 1950, National Security Council Report 68 warned that Soviet acquisition of atomic weapons heralded an irreversible shift in geopolitical momentum in Moscow’s favor. A few years later, Sputnik’s launch led many to fear that Soviet premier Nikita S. Khrushchev’s pledge to “bury” Western capitalism might just come true. President John F. Kennedy reportedly believed the USSR would eventually be wealthier than the United States, and Richard Nixon famously opined that America was becoming a “pitiful, helpless giant.” Over the next decade or so, defeat in Indochina and persistent economic problems led prominent academics to produce books with titles like America as an Ordinary Country and After Hegemony.¹ Far-fetched concerns about Soviet dominance helped propel Ronald Reagan to the presidency and were used to justify a major military buildup in the early 1980s. The fear of imminent decline, it seems, has been with us ever since the United States reached the zenith of global power.

Debates about decline took on new life with the publication of Paul Kennedy’s best-selling Rise and Fall of the Great Powers, which famously argued that America was in danger of “imperial overstretch.” Kennedy believed Great Britain returned to the unseemly ranks of mediocrity because it spent too much money defending far-flung interests and fighting costly wars, and he warned that the United States was headed down a similar path. Joseph Nye challenged Kennedy’s pessimism in Bound to Lead: The Changing Nature of American Power, which sold fewer copies but offered a more accurate near-term forecast. Nye emphasized America’s unusual strengths, arguing it was destined to be the leading world power for many years to come.

Since then, a host of books and articles—from Charles Krauthammer’s “The Unipolar Moment,” G. John Ikenberry’s Liberal Leviathan and Niall Ferguson’s Colossus to Fareed Zakaria’s The Post-American World (to name but a few)—have debated how long American dominance could possibly last. Even Osama bin Laden eventually got in on the act, proclaiming the wars in Iraq and Afghanistan fatal blows to American power and a vindication of al-Qaeda’s campaign of terror.

Yet for all the ink that has been spilled on the durability of American primacy, the protagonists have mostly asked the wrong question. The issue has never been whether the United States was about to imitate Britain’s fall from the ranks of the great powers or suffer some other form of catastrophic decline. The real question was always whether what one might term the “American Era”was nearing its end. Specifically, might the United States remain the strongest global power but be unable to exercise the same influence it once enjoyed? If that is the case—and I believe it is—then Washington must devise a grand strategy that acknowledges this new reality but still uses America’s enduring assets to advance the national interest...