Christopher Layne is professor and Robert M. Gates Chair in National Security at Texas A & M University’s George H. W. Bush School of Government and Public Service. His current book project, to be published by Yale University Press, is After the Fall: International Politics, U.S. Grand Strategy, and the End of the Pax Americana.

When Great powers begin to experience erosion in their global standing, their leaders inevitably strike a pose of denial. At the dawn of the twentieth century, as British leaders dimly discerned such an erosion in their country’s global dominance, the great diplomat Lord Salisbury issued a gloomy rumination that captured at once both the inevitability of decline and the denial of it. "Whatever happens will be for the worse," he declared. "Therefore it is our interest that as little should happen as possible." Of course, one element of decline was the country’s diminishing ability to influence how much or how little actually happened.

We are seeing a similar phenomenon today in America, where the topic of decline stirs discomfort in national leaders. In September 2010, Secretary of State Hillary Clinton proclaimed a "new American Moment" that would "lay the foundations for lasting American leadership for decades to come." A year and a half later, President Obama declared in his State of the Union speech: "Anyone who tells you that America is in decline . . . doesn’t know what they’re talking about." A position paper from Republican presidential candidate Mitt Romney stated flatly that he "rejects the philosophy of decline in all of its variants." And former U.S. ambassador to China and one-time GOP presidential candidate Jon Huntsman pronounced decline to be simply "un-American."

Such protestations, however, cannot forestall real-world developments that collectively are challenging the post-1945 international order, often called Pax Americana, in which the United States employed its overwhelming power to shape and direct global events. That era of American dominance is drawing to a close as the country’s relative power declines, along with its ability to manage global economics and security.

This does not mean the United States will go the way of Great Britain during the first half of the twentieth century...