Greg Djerijian asks whether America's "unipolar moment" is waning. The real issue here is that the unipolar moment prophesied by Charles Krauthammer in 1990 was always somewhat illusory. The true unipolar moment came not in 1990, but in 1946, when the United States enjoyed something like half of the world's economic output and a monopoly on nuclear weapons. Since that time, the long-term trajectory of US relative power had been distinctly downward. The collapse of the USSR bumped it back up, but not back to anything resembling 1946 levels and didn't alter the long run trajectory. Nor should this be a surprise -- sustaining the 1946 status quo would have been impossible and would have entailed miring the vast majority of humanity in a permanent state of economic misery. It's worth remembering that Robert Keohane's book on what happens to U.S. foreign policy After Hegemony was published in 1984 and locates the end of hegemony in the late 1960s and early 1970s.
For various reasons, by the dawn of the 21st century we'd arrived at a situation where only the United States maintained a serious capacity to "project" military power to regions distant from its borders. Nevertheless, a great many countries containing a majority of the world's people could not realistically be subjected to direct military coercion of this sort. In terms of non-military forms of power (including the coercive "hard power" of economics) the United States had long been the strongest player, but not strong enough to generate its preferred outcomes without cooperation from other major players. There's a reason, after all, while real or contemplated US military interventions during the so-called post-1990 "unipolar moment" have been concentrated in the Middle East and Africa -- these are the regions where there are no particularly strong local players so the sharp divergence in the great powers' ability to project force becomes decisive.