First of all: Are you sure you mean the early 19th century? In what ways do you think the United States behaved as an imperial power back then?

If you don't mind me asking: Is this a homework/essay question? If yes, what do you got so far? (We are willing to help.)

Just did a week on this in uni so ill link the sources we were given which covers it rather in depth. If you are a uni student you should be able to access them through the resource system of your university, my one calls it OneSearch but i have no idea what yours would be called.

Enforcing transnational white solidarity : Asian migration and the formation of the U.S.-Canadian boundary Chang, Kornel. In: American quarterly [2008]

Black and blue on San Juan Hill Kaplan, Amy. In: Cultures of United States imperialism [1993]

The problem of male degeneracy and the allure of the Philippines Hoganson, Kristin L. In: Fighting for American manhood : how gender politics provoked the Spanish-American and Philippine-American Wars [2000]

Copying and pasting those into the search (maybe sans date) should be sufficient.

The sources basically detail American Imperialism to be related heavily to American Exceptionalism, Masculinity and the Monroe Doctrine. I didnt handle the Monroe Doctrine part of the assignment but in regards to American Exceptionalism and Masculinity I cna speak on it.

So with Masculinity you get men like President Ted Roosevelt and Senator Beverage who wanted war since the American man in their eyes was becoming weak and complacent due to the advances of civilization. The men in the Phillipines as Roosevelt saw them were prime examples of masculinity and in fighting them the American man could strengthen himself while imparting civilization to the "savages". So the wars were in part in relation to the idea that the American man needed to be stronger and Roosevelt saw this as being possible through war.

In regards to American Exceptionalism it can be simply said to be “Human Rights Narcissism" as noted by Michael Ignatieff in his essay. This can be directly related to Cuba where the rebels had pretty much won the war but the Americans saw the waning Spanish influence as a chance to expand. Since Cuba was so close, the Americans saw Cuba as "rightfully theirs" regardless of rebels fighting and dying for their freedom. With various legislation after the fighting the Americans pretty much took over Cuba as a colony forcing them to go through America if they wanted to exist on a global stage.

Use your library resources and just type "American Exceptionalism" and you will get several thousand hits easily.