American imperialism is the economic, military and cultural philosophy that the United States affects and controls other countries. Such influence is often closely associated with expansion into foreign territories. The concept of an American Empire was first popularized during the presidency of James K. Polk who led the United States into the Mexican–American War of 1846, and the eventual annexation of California and other western territories via the Treaty of Guadalupe Hidalgo and the Gadsden purchase.
Thomas Jefferson, in the 1790s, awaited the fall of the Spanish Empire "until our population can be sufficiently advanced to gain it from them piece by piece." In turn, historian Sidney Lens notes that "the urge for expansion – at the expense of other peoples – goes back to the beginnings of the United States itself." Yale historian Paul Kennedy put it, "From the time the first settlers arrived in Virginia from England and started moving westward, this was an imperial nation, a conquering nation." In a 2008 address to Boston University, Noam Chomsky stated that "talking about American imperialism is rather like talking about triangular triangles." Detailing George Washington's description of the early United States as an "infant empire", Benjamin Franklin's writing that "the Prince that acquires new Territory ... removes the Natives to give his own People Room... may be properly called [Father] of [his] Nation," and Thomas Jefferson's statement that the United States "must be viewed as the nest from which all America, North & South is to be peopled," Chomsky stated, "the United States is the one country that exists, as far as I know, and ever has, that was founded as an empire explicitly."
Stuart Creighton Miller says that the public's sense of innocence about Realpolitik impairs popular recognition of U.S. imperial conduct. The resistance to actively occupying foreign territory has led to policies of exerting influence via other means, including governing other countries via surrogates or puppet regimes, where domestically unpopular governments survive only through U.S. support.