Thought this might provide some interesting reading given the subject.
From the Oxford Concise Dictionary of Politics (I know dictionaries often come up short, but this one's pretty good most of the time):
" Deriving from the Latin term (imperator) for a supreme military and, later, political leader, empire came to mean a territorial realm over which exclusive authority was exercised by a single sovereign. Thus the preamble of the English Act of Appeals (1533) justified denial of the right of subjects of the Crown to appeal to courts outside the realm or territory of England on the ground (however dubious) 'that this realm of England is an empire, and so hath been accepted in the world, governed by one supreme head and king'.
The term soon came to be applied to the much more loosely controlled and heterogeneous domains of princes such as the Hapsburg Emperor Charles V, even when his power was manifestly copmromised and limited in many places, and most of all in the so-called Holy Roman Empire from which he derived the title, by continuing privileges of the Church, lesser princes, cities, guilds, electors, and estates. Likewise, Queen Victoria adopted the style of Queen Empress in 1877 at precisely the moment when the addition of India and new African dependencies to her dominions led them to resemble the ramshackle constitutional amalgams of her Austrian and Russian cousins more than the older English ideal of a contiguous territory with a homogeneous population. Thereafter, 'empire' was generally taken to denote an extensive group of states, whether formed by colonization or conquest, subject to the authority of a metropolitan or imperial state, even when - as in France or the USSR - that dominant state became a republic lacking an emperor or empress at its head. In this later sense, well established by the early years of the twentieth century, empire became closely associate with 'imperialism'."
Imperialism:
" Domination or control by one country or group of people over others, in ways assumed to be at the expense of the latter. Beyond this sweeping definition, there is much disagreement over the precise nature and the causes of imperialism, about what the clearest examples are, about its consequences, and therefore over the period which exemplifies it best.
The so-called new imperialism pertains to the imposition of colonial rule by European countries, especially the 'scramble for Africa', during the late nineteenth century. Many writers have construed imperialism in terms of what they believe were the motivating forces behind the territorial expansion. Among these, Hobson, Luxemburg, Bukharin, and especially Lenin focused on economic factors, such as the rational pursuit of new markets and sources of raw materials. The last named argued, in
Imperialism: The Highest Stage of Capitalism (1917), that imperialism is an economic necessity of the industrialized capitalist economies, seeking to offset the declining tendency of the rate of profit, by exporting capital in the pursuit of investment opportunities overseas. For Lenin, imperialism is the monopoly stage of capitalism.
In a very different theory, Schumpeter (1919) defined imperialism as the non-rational and objectless disposition on the part of a state to unlimited forcible expansion. Imperialism is rooted in the psychology of the rulers and the effects of surviving pre-capitalist social structures, not the economic interests of nation or class. Yet other accounts view imperialism as an outgrowth of popular nationalism, a function of the need to underwrite the welfare state which helps pacify the working class (notably in Britian), a matter of personal adventurism, an application of Social Darwinism to struggles between races, a civilizing mission, and as simply one dimension of international rivalry for power and prestige. The latter in particular means that imperialism is potentially a feature of leading socialist as well as capitalist states.
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The concept of 'informal imperialism' is said to render direct political control unnecessary, in the presence of other ways of exercising domination, for example through technological superiority or the free trade imperialism of a leading economic power, and cultural imperialism."
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This imperialism without colonies was first characterized by Ghana's first President, Kwame Nkrumah... 'The essence of neo-colonialism is that the State which is subject to it is, in theory, independent and has all the outward trappings of international sovereignty. In reality, its economic system and thus its political policy is directed from outside.' "
I wouldn't personally characterize America as a formal empire but I would put it under the "informal empire" by virtue of its "informally imperialist" (not so informal at times) posture and level of control over many countries.
The only way to creat a rapid decline in American power is "Imperial Overstreach", and a series of destructive wars such as those the Spanish fought in the 1500s, the tow World Wars which drained the British Empire or the Cold War, which ate the economy of the Soviet Union. A war with China would probably be the only event on a big enough scale to count (only 10-12% of the US Armed Forces are engaged in Iraq and Afghanistan, so there is still a lot of streach there), either a hot war or a long and grinding Cold War.
Good point, though I disagree that "Imperial Overstretch" must have military overstretch as a facilitator (which I think you were getting at, though I may be wrong). I think political overstretch can do wonders where military overstretch may not be reached. By political overstretch I mean an extension and employment of power beyond what the system (international) and its actors can tolerate. I think military action can cause it, but not necessarily because that military action constitutes military over-extension but rather that the military action asserts power in such a way that, though the power is temporarily expanded, the problems it creates eventually cause a decline rather than an overall growth as their persistence is greater than that of the advantage gained. Combine that with the "balancing and bandwagoning" (an overly simplistic characterization but a relatively accurate one nonetheless, I think) tendencies of states and all that wonderful complex interdependence going around and I think I'd have to wager that American power is more likely to decline over the next few decades, especially with the growth and expansion of non-state actors like the EU, NAFTA, FTAA, East Asia bloc, etc.