The second option (
The United States became an imperialist nation) is correct.
Explanation:
American imperialism arose from the combination of two factors. First, economic causes: its spectacular economic growth, especially after the end of the civil war, provided the United States with great military power and generated the need to seek markets and more raw materials. Second, US foreign policy received a clear influence of social Darwinism, that is, Darwin's theory of survival and evolution in the political sphere of international relations.
