At the time the Ostend Manifesto was written, Pierre Soulé was the U.S. Minister to Spain from 1853-1855. He was a former U.S. Senator from Louisiana. Although he was born and raised in France, Soulé was exiled for revolutionary activities and later moved to the United States.

      He was one of the three men at the secret meeting in Ostend, and is credited with being the mastermind behind the manifesto's doctrines, but not the physical writer. He supported the Southern expansion of influence into the Caribbean and Central America, and therefore agreed to be the main contributor to the Ostend Manifesto's policies. He is known to be the aggressive voice in the document and used "Gunboat Diplomacy," which is foreign policy backed by the threat of military force.

     After the Ostend Manifesto was publicized, he came under heavy personal criticism for violating his role as a Minister to Spain, since they still had control over Cuba.