President James Monroe presented his famous doctrine for the western hemisphere in 1823 . Historians have observed that while the Monroe Doctrine contained a commitment to resist colonialism from Europe , it had some aggressive implications for American policy , since there were no limitations on the US 's own actions mentioned within it . Scholar Jay Sexton notes that the tactics used to implement the doctrine were " modeled after those employed by British imperialists " in their territorial competition with Spain and France . Eminent historian William Appleman Williams dryly described it as " imperial anti-colonialism . " The Indian Wars against the indigenous population began in the British era . Their escalation under the federal republic allowed the US to dominate North America and carve out the 48 continental states . This is now understood to be an explicitly colonial process , as the Native American nations were usually recognized as sovereign entities prior to annexation . Their sovereignty was systematically undermined by US state policy ( usually involving unequal or broken treaties ) and white settler-colonialism . The climax of this process was the California genocide
California genocide
California Genocide
American imperialism