Although pro-annexation sentiments had grown more popular during the Polk administration, when Texas was annexed to the United States through military force, groups within the U.S. had opposed incorporating Texas into the U.S. for fear of inciting the slavery firestorm. The concern was that rival factions would rip each other apart over the spread of slavery. Former President Andrew Jackson, for example, had opposed annexation on the premise that it would inflame the already volatile slave issue.

O'Sullivan, however, understood slavery as peripheral and of little relevance to his larger picture of manifest destiny, which O'Sullivan saw as the obvious future of the United States. He believed it was foreordained that the "anglo-saxon" settlers of the U.S. would gradually take over the entire North American continent. He based this, ignoring slavery, on the idea that the U.S. was a future-oriented, vanguard nation that had ripped off the yokes of oppression and inequality that held Europe down. He argued that because of the United States's deep belief in liberty and equality, and the way its European heritage and ethnicity made it obviously culturally superior to native groups, it was inevitable, at least to him, that U.S. ideas and culture would take over geographically. He downplayed slavery because he believed it was just a distraction from the larger issue, and he didn't want it interfering with U.S. expansion.

O'Sullivan suggested as well that, rather than inflame slavery politics in the U.S., Texas could become a gateway through which the U.S. could "slough...off" the black population to Central and South America.

John O’Sullivan wrote about why the United States should expand from the Atlantic Ocean to the Pacific Ocean. He wanted to be sure that the people of the United States understood the reasons for the policy of Manifest Destiny, and why it had nothing to do with the expansion of slavery.

In his essay, published in 1845, O’Sullivan emphasized how the cause of the United States was a noble one. He discussed how the United States had furthered the cause of liberty and the cause of oppressed people. He stated that once the United States became independent that there had not been horrible, destructive wars fought on the soil of the United States. As a result, the death and carnage associated with those wars is not something that was connected with the land that made up the United States. The United States had furthered the cause of liberty. He argued that these factors showed that it was the duty of the United States to expand to the Pacific Ocean.

By not tying expansion to slavery, the policy of Manifest Destiny would be one that all Americans could support. He stressed that the equality of rights is one of the basic missions of the United States. As a result, he believed the United States should expand, and that all Americans should support this goal of expanding to the Pacific Ocean.