Was imperialism a legitmate policy for the united states to follow at the turn of the nineteenth century?
That depends. Do you consider the policy of imperialism legitimate at any time? Most don’t. At the time, America was strong, fast-growing, and ambitious. We were trying very hard to get ourselves into a position that could not be harmed by another country, mainly Britain, who we’d just had a major war with, and would soon have another (1812, which is after the turn of the 19th). We were trying not only to get the goods and thus money that would come out of the lands we claimed, but also create a sparsely populated buffer zone that an enemy would have to cross before they could get to us, which would give the army time to mobilize. They considered legitimate by giving the reason that they, as white Christians, were entitled by God to claim the lands within reach. They were allowed, by God’s Word, to “cleanse” those lands, and rid it of heathens, whether by conversion or killing. Most don’t consider that legitimate nowadays.