O'Sullivan and other promoters of what became known as "manifest destiny" argued that the United States, as a modern, democratic, Christian nation had the God-given right to expand throughout the continent. O'Sullivan, and indeed most of his contemporaries, associated modernity, democracy, and even Christianity with whiteness. Manifest destiny by its very nature was racist, essentially advocating that white people were entitled to take land that belonged to the Mexican people. O'Sullivan and others were also entirely dismissive of the idea that Native Americans could have a right to lands in the West.
O'Sullivan makes this his views on race explicit in "Annexation," his article urging the United States to annex Texas. O'Sullivan, like many other Americans, saw expansion as a means of ridding the United States of slavery (or, more accurately, African-American enslaved people.) He thought that if Texas were annexed, it would provide a "drain" through which slaves might be sold into Latin America, where they could labor more profitably and far away from the white race that he thought should dominate the United States:
The [people] of Mexico, Central America, and South America furnish the only receptacle capable of absorbing that race whenever we shall be prepared to slough it off...Themselves already of mixed and confused blood...
Most white Americans, whether or not they embraced manifest destiny, would have held similar views on race. But manifest destiny as an ideology was based on the notion that the destiny of the United States was one to be realized and enjoyed by whites only.