The doctrine of Manifest Destiny was a belief that the imperialistic expansion of conquered lands was not only the right of each nation but also the duty of that nation. In the United States, this was demonstrated through the settling of the western frontier. This imperialist and selfish rationalization does not coincide with the spiritual and humanistic beliefs of Walt Whitman, and particularly with the way he portrays himself in "Song of Myself".
In this poem, Whitman is espousing on the interconnected status of all humans, both with each other and with the spiritual and natural world. The third line alone completely contradicts Manifest Destiny:
"For every atom belonging to me as good belongs to you."
If we as humans share everything, then no one can claim to own land, can they? Consider this passage:
"Have you reckon'd a thousand acres much? have you reckon'd the earth much?
Have you practis'd so long to learn to read?
Have you felt so proud to get at the meaning of poems?
Stop this day and night with me and you shall possess the origin of
Again, there is the feeling of shared experience and shared wealth. The thousand acres are only a part of the earth. The poems individually are only a product of all poems, and we each "shall possess" them.
Whitman may not have been specifically contradicting the popular theory, but he was certainly clear not to celebrate material wealth, in any form.