Ostend Manifesto, (October 18, 1854), communication from three U.S. diplomats to Secretary of State William L. Marcy, advocating U.S. seizure of Cuba from Spain. The incident marked the high point of the U.S. expansionist drive in the Caribbean in the 1850s.

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After Pierre Soulé, U.S. minister to Spain, failed in his mission to secure the purchase of Cuba (1853), Marcy directed James Buchanan, minister to Great Britain, and John Y. Mason, minister to France, to confer with Soulé at Ostend, Belgium. Their dispatch urged U.S. seizure of Cuba if the United States possessed the power and if Spain refused the sale. This action stemmed both from fear of a slave revolt in Cuba similar to that in Haiti and from a desire to expand U.S. slave territory. Their proposals, couched in intemperate language, were rejected, and, when contents of the dispatch leaked out, the Republican press branded it as a “manifesto” appealing to Southern opinion.