The Ostend Manifesto was a secret document written by American diplomats in 1854 at Ostend, Belgium. The manifesto outlined a plan for the United States Government to acquire the island of Cuba from Spain. Located only 150 miles from Miami Florida, many American expansionalists believed the America had the "right" to Cuba.

The diplomats, Pierre Soule, James Mason, and James Buchanan, were all staunch advocates of slavery and expansion. They threatened to obtain Cuba by force if Spain refused to sell the island for $120 million. Soule, in particular, proved extremely antagonistic and was said to have spearheaded the threatening tone of negotiateions. The threat, which the diplomats were not authorized to make, soured relations between the United States and Spain and resulted in immediate rejection in England, France, and Spain. The United States Secretary of State, William Marcy was forced to unconditionally repudiate the manifesto. When word of the manifesto leaked, it created a great controversy in the northern states. Because the diplomats were well-known advocates of slavery, Northern politicians and abolitionists expressed outrage and decried the manifesto as an attempt to extend slavery. Southerners generally advocated the manifesto because many believed that Cuba would become an independent Black republic.

The incident further strained relations between politicians in the North and the South, and brought the nation one step closer to Civil War.