At the time the Ostend Manifesto was written, James Buchanan was the U.S. minister to Great Britain (appointed by Pres. Franklin Pierce). He was previously Secretary of State under James Polk. Buchanan was pro-slavery, and tended to support the South in most of their decisions in Congress. Although he believed slavery was morally wrong, he also thought that freeing them was a greater evil.

     He was one of the three men at the secret meeting in Ostend, and is credited with the actual writing of the document, aided by Soulé and Mason. His motive for writing the Manifesto stemmed from his strong belief in American imperialism. In his message to the Senate in 1858, Buchanan stated, "It is beyond question the destiny of our race to spread themselves over the continent of North America, and this at no distant day should events be permitted to take their natural course."