Question

why was the united states justified in their imperialistic policies of the late 1800s and the early 1900s ? why did the united states shift from expanding internally towards foreign expansion in the late 1800s and early 1900s?

Answer 1

This question contains assumptions which are not historically accurate. The United States has never practiced imperialism, and there never was any "foreign" expansion. No one can credibly claim, for example, that the annexation of Hawaii in 1898 differed in any serious way from the annexation of Texas in 1845. Either can be regarded as "foreign" expansion, since in both cases the territory in question had an existing government -- in the case of Hawaii, its traditional native rulers, and in the case of Texas, the Republic of Mexico. Even the expansion of the United States prior to the Mexican War is hardly expansion into a vacuum: the land of the Northwest Territory and Louisiana Purchase were filled with native American nations. There never was any such thing as "internal" expansion: every acre of US expansion involved displacement or subjugation of its prior rulers.

In any event, the question confuses territorial acquisition with acquisition of empire. Imperialism does NOT consist of acquisition of territory added to the home nation. An empire consists of *several* nations bound together with common foreign policy and close trading ties, and often dominated by one of them, but where the several nations retain their individual identities and internal distinctions. The British Empire was an empire because the British did NOT annex India, Canada, Australia, and New Zealand and make them part of England: instead all those nations retained their separate identities and internal governments, but coordinated foreign policy and trade agreements.

There has never been such a system containing the United States. The closest you can come is the "paternalism" of the American "stewardship" of the Phillipines, between their existence within the Spanish Empire and independence, the various weird little UN-sanctioned oversight of tiny Pacific Islands, and the odd situation of Puerto Rico, which seems to enjoy its status as a US dependency (Puerto Rican voters routinely turn down attempts at statehood, presumably because it would involve paying Federal taxes).