Who were the American Anti-Imperialists? What were their arguments against imperialism? Did some of the Anti-Imperialists make arguments against overseas expansion based on racist views? What was the basis of other arguments against American expansion in the Pacific? What were the causes of the Spanish-American War in 1898? How did American interest in the Pacific contribute to the outbreak of war with Spain? Did the United States make the correct decision in going to war against Spain, or was it an unnecessary war?

This is a very broad set of questions that could honestly cover an entire history class, so I'll just focus on the first few. The American Anti-Imperialist League was founded in 1898. The former Secretary of Treasury George S. Boutwell was president of the League, whose ranks touted a number of notable men, including Mark Twain and Andrew Carnegie. Initially, the League formed to fight against the annexation of the Philippines based on "economic, legal, and moral grounds." In a group of educated white men in the nineteenth century it is of course expected that some were against overseas expansion due to racial views, fearing "an influx of non-Whites." However, it is important to note that many members were involved in the abolition movement and the League was not, as a whole, entirely against all overseas expansion. Isolationism in overseas affairs was an undercurrent in American politics all the way up until the World Wars, so it's not surprising that some members could be against overseas expansion purely based on isolationist views.
Prominent members, including Edward Atkinson, Moorfield Storey, and Grover Cleveland, believed imperialism "undermined democracy at home and abroad and violated the fundamental principles upon which America had been founded." The subjugation of other peoples was highly unmoral and unwarranted aggression on the behalf of a government that should be fueled by the explicit consent of the governed. Imperialism, then, would not be an acceptable form of government. America has a long history of intervention meant to colonize disguised under a sheen of redeeming democracy. The first World War was opposed by the League for turning into a war not for humanity, but for empire. However, by 1921, the League had disbanded after losing power and influence for years. Their morals and beliefs hold an eerie sense of foreshadowing for American overseas politics in the next century, and learning their views and protestations is incredibly important in this politically fraught era.
http://www.loc.gov/rr/hispanic/1898/league.html

https://www.newworldencyclopedia.org/entry/American_Anti-Imperialist_League