One of the most controversial issues facing Americans in the late nineteenth and early twentieth centuries was the question of imperialist expansionism. The dilemma of whether or not the taking of colonies or the annexation of territories beyond the bounds of the North American land mass was congruous with previous American policies, including the Declaration of Independence, remained a major foreign policy debate between the Spanish-American War, beginning in 1898, and World War I, beginning in 1914. The truth of the matter was that expansion beyond the North American land mass was in fact a departure from earlier procedures but it was a departure that many Americans considered justifiable.
The first, and clearest, indication of a drastic change of policy was the Spanish-American War. American intervention between Spain and her colonies of Cuba and the Philippines constituted involvement in European affairs that had previously been avoided under the Monroe Doctrine which was in turn based on Washington's Neutrality policy. Still more controversial was the Treaty of Paris of 1898 through which the United States bought the Philippines and took control of a people that did not want to be ruled by them.
There are several ways in which imperialism differed from the conquest of the American West and Mid-West. Significantly, until the American "invasion" of what was to become the western portion of the United States, those lands were only sparsely populated by Native Americans. Thus, white conquest of the region involved no conflict with other countries, such as European nations, that colonization of other parts of the world was sure to entail. The American policy toward Native Americans had largely been to deny them citizenship and enclose them in small sovereign reservations (excluding the failed attempts at assimilation). Such a policy would not be applicable in an imperialist system.