When the Ostend Manifesto was written, William L. Marcy was the U.S. Secretary of State under President Franklin Pierce. He also served as the 20th U.S. Secretary of War (1845-1849) for President James Polk. Later in his career, Marcy negotiated the Gadsden Purchase from Mexico, which added land to what are now the states of New Mexico and Arizona.

     After the Pierce administration had been pressured by Southern expansionists to consider buying Cuba, Marcy suggested that Buchanan, Soulé, and Mason secretly meet and make a plan to attempt to buy Cuba from Spain. After the document accidentally became publicized, the response was all but positive, especially towards the people involved. Marcy and the Pierce administration tried to disassociate themselves from the Ostend Manifesto, but the damage had been done.