Question

O'Sullivan argued that American expansion and imperialism was justified because

Answer 1

 O' Sullivan believed that the United States had a mission to expand, spreading its form of democracy and freedom

Answer 2

John Louis O'Sullivan was an American author and editor who first coined the term "Manifest Destiny." He was a promoter of the annexation of Texas and the Oregon Country to the United States.

The idea of "Manifest Destiny" was the belief that it was the duty of the United States (or, its destiny) to expand across the whole continent. This motivated many political and social events, such as the expansion to the West. O'Sullivan believed that this justified expansion and imperialism.