I might have guessed as much after a situation a couple of years ago in which my colleague Trevor Getz and I were asked to debate this topic before a high school history club (just to give them the (enjoyable?) experience of watching a couple of historians argue with each other and demonstrating that history was far more than the simple narratives most high school history texts offer).
In that debate, I drew the “short straw,” and argued that the US was not an empire. It’s a tough case and depends mostly on semantics and the difference between the US experience and that of the “classic” empires of conquest and territorial expansion (e.g. Rome, Mongols, Spain, Portugal, England, France, etc.). With the exception of the notable but relatively insignificant seizure of the Spanish possessions (Philippines, Cuba, and a few other islands) at the end of the 19C, and the less visible but more extensive and brutal ejecting of indigenous peoples across the continent from the 16C through 20C, the US has not set up a network of overseas colonies in the European style.
Rather, since the US only developed the ability to project power across long distances at the end of the 19C and Europeans had already begun losing faith in their “new imperialism” model by that time, the nature of imperialism globally since then has been of the “informal” variety. Instead of running up flags and establishing local governments directed from metropolitan capitals (London, Paris, Washington), “informal” empires save much of the cost of administration and military defense in favor of the exercise of economic and cultural power by imperial governments, companies, banks, and other institutions.
From the home country (the “metropole”) this sort of operation looks a lot less like an empire than the old-style European model. This shift also fit the US self-image of being anti-imperialist (we did throw off the British yoke back in the 18C, after all) and our national mythology of less government and being the global champion of “freedom.”
In the first part of the 19C, the British led a campaign to open up national/imperial economies, under the battle cry of “free trade.” It was a fitting effort by what was becoming the dominant global economy to ensure that all markets were open to British intervention; a self-serving ideology of “freedom.” So, too, the US in the 20C opposed European (& Japanese) imperialism (see, e.g., Wilson’s efforts at the post-WWI Peace Conference and Roosevelt’s parallel effort as WWII was winding up). In both cases, “openness” meant opportunity for the merchants and financiers and the religious and other cultural entrepreneurs of the rising global power to establish themselves to the disadvantage of local and imperial competitors.
The “home” empires of the US and Russia consisted of seizing and incorporating land and indigenous peoples into an expanding metropole. This was larger in scope but along the same lines as what England did to Wales/Scotland/Ireland or Paris did to Aquitaine and Burgundy in earlier centuries.
The “new style” of empire, pioneered by the British in Latin America in the early 19C (and, arguably, in India even earlier) eschewed formal control. Funding, trade contracts, an occasional gunship, and a comment or two from an “advisor” to the local elites would suffice. These empires were not “state-to-state,” but “society/economy-to-society/economy.” Barclay’s Bank presaged Coca-Cola as the vehicle of power and profit. Commercial and cultural empires also avoided messy issues of national coherence and whether to include large colonial populations into the metropolitan polity (only white “Frenchmen” from Algeria could vote in France’s elections, even if Algeria had been formally part of France since 1830; and no one could contemplate hundreds of millions of Indians voting for Gladstone or Disraeli or Churchill). Companies viewed governmental empires as only occasionally useful tools.
By the mid-20C, military technology reduced the need to hold (and need to defend!) vast swaths of distant territories at considerable cost. A few air force and navy bases would do the job. Companies and non-governmental organizations could be paid (sometimes even openly) to advance the interests of the distant power center: the IMF could manage the budgets of entire countries, local groups could be subsidized to promote “democratic” values?
Perspectives from “colonized” peoples were not so different from one style to another. Geopolitical power is pretty hard to disguise after a while. It’s clear when you’re not even running your own country, despite the presence of local parties and Western-certified elections. Similarly, for Americans overseas, expat colonies, widespread use of English, Holiday Inn’s in Tibet and McDonald’s franchises in Java made it seem an “all-American” world, regardless of whose flag was flying.
It's good to remember all this, even in an age when US world-wide dominance is waning. There is no inherent reason China should not be a bigger global power than the US. They are 3-1/2 times more populous after all. Furthermore, the track record of prior hegemons shows that incumbents, sooner-or-later, get too fat-and-happy to stay at the top of their game. Finally, no one should confuse any of this with what is “right” or other morality claims/self-justifications.
So, other than the sharp variance from the Spanish/British model, the case for the US being an imperial power is pretty strong, regardless of our self-proclaimed ideology/mythology. We still project an awful lot of power around the world—from aircraft carriers to Facebook to financial infrastructure. It’s far from the whole story of the US in the world, but this power does polarize the perspectives of both the subjects of that power and our own.