A model advocated by international relations
theorist Samuel Huntington
to explain exactly what's going on in the new world order
that followed the Cold War
. His idea received tenuous treatment in the book The Clash of Civilizations and the Remaking of World Order
, and was fleshed out in greater detail in a 1999 Foreign Affairs
article entitled "The Lonely Superpower
In essence, the unimultipolar model is a hybrid of two other models that cropped up after the fall of the USSR. The unipolar world model placed the USA as a single, uncontested superpower in the world order, akin to the Roman Empire in Europe two millenia ago. The multipolar world model, on the other hand, elevated China, the European Union, and sometimes other entities to superpowerdom alongside the United States.
Huntington argues that the multipolar model was clearly wrong. No other state has the global reach of the United States, and the few states that trump the US in manpower fall far behind it in the technology they wield. However, with the exception of a brief honeymoon period around the time of the first Gulf War, the US has never been truly uncontested for power in the international arena. So, Huntington combined the two models to create a "unimultipolar" model.
In the unimultipolar model, to quote Huntington's article, "the settlement of key international issues requires action by the single superpower but always with some combination of other major states; the single superpower can, however, veto action on key issues by combinations of other states." He argues that while the United States exerts its power on a global macro-sort-a scale, its power is supplemented by other countries that are strong in their own neck of the woods, but not necessarily to the same extent as the USA. Huntington cites the "condominium" of Germany and France, Russia, China, Iran, India, Brazil, Nigeria, and South Africa as the second-tier powers. These second-tier powers are often contested within their own spheres of influence by other powers, such as the United Kingdom, Japan, South Korea, Pakistan, Saudi Arabia, and Argentina.
While the US often tries to act like the sole key actor on the international scene, it really isn't. American actions, such as George W. Bush's proposed invasion of Iraq, have to meet the approval of other powers as well: otherwise, they become "unilateral" actions and lack the political momentum necessary to be effective. Even American economic sanctions fail to unseat regimes unless they include the support of other powers.
The historical basis of this situation has its roots in the conflict of the Cold War. At that time, most countries were looking for a nuclear umbrella to fall under, and many were happy to coalesce with the American bloc in opposition to the communist bloc, which appeared, at the time, to be a formidable threat. Under the Truman Doctrine, the US was willing to pump massive amounts of aid into these countries to strengthen them against the communist menace and the purported domino effect. Once the Soviet threat had passed, America still saw itself as dominant over these countries, but the main motive behind that relationship no longer existed, making the hegemony pointless to the lesser states (aside from being an economic boon to America).
Washington, D.C. maintains its global hegemon status because there have been no successful attempts at creating a balance of power against the US. This is partly because too many countries are benefitting from aid and trade vis a vis America, but is also largely because any successful counter-alliance would cross deep civilizational lines and collapse out of cultural differences and mutual distrust. The world of Islam, for example, is unlikely to ever build a healthy alliance with China or Russia, simply because each would be afraid of the other's cultural values taking predominance. NATO and the Warsaw Pact owe much of their admittedly limited success to the fact that they were dealing with fairly solid religious and linguistic blocs of nations.
For all sorts of detail on this model, I urge you to read Huntington's original article: http://www.ub.edu.ar/facultades/feg/lonely_superpower.htm