Imperialism was such a powerful topic in the late nineteenth-century because it represented how America viewed itself at the time. The United States was once a colony itself and many people thought that the nation should not own colonies of its own. Colonial ventures were expensive, as one had to subdue the land to be colonized as well as establish infrastructure there. There were also some people who missed having a "frontier" to discover. Frederick Jackson Turner thought the closing of the West meant that the United States would lose its pioneer ethos and the American cultural identity would change for the worse. There were also many events that pushed imperialism to the forefront in the late nineteenth-century, mainly the Spanish-American War and the fate of the new American acquisitions from that conflict.
The imperialists argued that the United States could export its culture and value system to the developing world. They also argued that the nation needed to ensure coaling stations all over the world to maintain its shipping lanes. This was especially important as the United States sought more Pacific trade during this time period. The imperialists also wanted to gain raw materials from the developing world before European countries colonized these places.
Anti-imperialists thought that American expansion would divert interests and money from a domestic Progressive agenda. They also thought that America would lose its moral high ground by grabbing land. There was already some concern that colonies caused a country to lose this moral high point as the United States regularly condemned British and Belgian actions in Africa. Some anti-imperialists protested the Filipino insurrection that took place after the Spanish-American War by claiming that the United States was being a bully toward a people seeking their freedom. Some anti-imperialists claimed that the new colonial territories would never become states and would thus be a drain on the nation in terms of building infrastructure. While your opinion may be different, I tend to take the side of the anti-imperialists. American acquisitions in the Pacific put the nation on a collision course with Japanese interests in the region. The United States overthrew the legitimate government of Hawaii in order to get out of paying tariffs on sugar and pineapple from the islands. The Spanish-American War was based on yellow journalism and the US was never threatened. The United States did not credit those Filipinos who helped win the islands away from the Spanish and the Philippines remained an American colony until after WWII. While there are valid reasons for taking the side of the imperialists, I tend to favor their opposition.