1826, "advocacy of empire, devotion to imperial interests," originally in a Napoleonic context, also of Rome and of British foreign policy; from imperial + -ism. At times in British usage (and briefly in U.S.) with a neutral or positive sense relating to national interests or the spread of the benefits of Western civilization, but from the beginning usually more or less a term of reproach. General sense of "one country's rule over another," first recorded 1878. Picked up disparagingly in Communist jargon by 1918.
It is the old story of 1798, when French republicanism sick of its own folly and misdeeds, became metamorphosed into imperialism, and consoled itself for its incapacity to found domestic freedom by putting an iron yoke upon Europe, and covering it with blood and battle-fields. [Francis Lloyd, "St. James's Magazine," January 1842]