What was one argument against nineteenth-century American imperialism?

Traditional American principles opposed colonization.

Explanation:

In the beginning of 1800s, Most of American people supported the Monroe Doctrine as their traditional principle. This doctrine heavily opposed colonization that conducted by European empires all over the word.

American imperialism is aimed to gained influence in other country by using its military power. This is very similar with colonization, it just has different terminology. Because of this, American imperialism actually is not suitable to the traditional American principle.