A memorandum draft resulted, called the Ostend Manifesto, which proposed the purchase of Cuba from Spain, then in the midst of revolution and near bankruptcy. The document declared the island "as necessary to the North American republic as any of its present ... family of states". Against Buchanan's recommendation, the final draft of the manifesto suggested that "wresting it from Spain", if Spain refused to sell, would be justified "by every law, human and Divine".[31] The manifesto, generally considered a blunder, was never acted upon. It weakened the Pierce administration and reduced support for Manifest Destiny.[31][32]

Presidential election of 1856
Main article: 1856 United States presidential election
1856 Map of electoral votes
Buchanan's service abroad allowed him to conveniently avoid the debate over the Kansas–Nebraska Act then roiling the country in the slavery dispute.[33] While he did not overtly seek the presidency, he assented to the movement on his behalf.[34] The 1856 Democratic National Convention met in June 1856, producing a platform that reflected his views, including support for the Fugitive Slave Law, which required the return of escaped slaves. 