The Collaborative International Dictionary of English v.0.48:

Anti-imperialism \An`ti-im*pe"ri*al*ism\, n.
   Opposition to imperialism.

   Note: This term was applied originally in the United States,
         after the Spanish-American war (1898), to the attitude
         or principles of those opposing territorial expansion;
         in England, of those, often called Little Englanders,
         opposing the extension of the empire and the closer
         relation of its parts, esp. in matters of commerce and
         imperial defense. After the second world war, the term
         became used for opposition to any hegemony of one power
         over a foreign territory, and to the support for the
         national independence of territories, as in Africa,
         which were controlled by European nations. --
         An`ti-im*pe"ri*al*ist, n. --
         An`ti-im*pe`ri*al*is"tic, a.
         [Webster 1913 Suppl. +PJC]