There were, of course, many people who were on either side of this debate. One of the most prominent people to be opposed to imperialism and expansion was the author Mark Twain.
Twain was opposed to expansion because he thought it was an affront to the values that America stood for. He believed that America should be about securing freedom and independence for all people. When Twain thought about imperialism, he realized that the US was going to be depriving places like the Philippines of their chance to rule themselves. Because of this, Twain and others like him in the Anti-Imperialist League opposed the US expansion that came after the Spanish American War.