We are in a protracted period of international transition, one that began more than two decades ago with the Cold War's end. That era of strategic rivalry between the United States and the Soviet Union gave way to one in which the US possessed far greater power than any other country and enjoyed an unprecedented degree of influence.
That American unipolar moment has given way to a world better described as non-polar, in which power is widely distributed among nearly 200 states and tens of thousands of non-state actors ranging from Al Qaeda to Al Jazeera and from Goldman Sachs to the United Nations.
But what distinguishes historical eras from one another is less the distribution of power than the degree of order between and within states. Order never just emerges; it is the result of conscious efforts by the most powerful entities in the world.
While the US remains the world's most powerful single country, it cannot maintain, much less expand, international peace and prosperity on its own. It is over-extended, dependent upon massive daily imports of dollars and oil, and its armed forces are engaged in demanding conflicts in Afghanistan and Iraq. The US lacks the means and the political consensus to take on much more in the way of global responsibility. It also lacks the means to compel others to follow its lead.