The Black Warrior was a merchant steamer that generally trekked between New York City and Mobile, often stopping at Havana, Cuba. On February 28 1854, the ship was boarded and seized by Spanish authorities at Havana. They imposed a $6,000 fine on the grounds that the ship had violated customs regulations and arrested the crew.
After the arrests, Spain and the U.S. found themselves close to waging war on each other. The South, anxious to annex Cuba in a last-ditch effort to save slavery, vigorously fanned the flames of war over this event, but the North refused.
Before it was over, three American diplomats (Pierre Soule, James Mason and James Buchanan -- U.S. ambassadors to France, Spain and Great Britain and all pro-slavery Democrats) held a meeting in Ostend, Belgium, on October 9, 10 and 11, 1854, where they drew up a manifesto that stated in part: "Our past history forbids that we should acquire the island of Cuba without the consent of Spain unless justified by the great law of self-preservation After we shall have offered Spain a price for Cuba, far beyond its present value, it will be time to consider the question; 'Does Cuba in the possession of Spain seriously endanger peace and our cherished Union?' Should this question be answered in the affirmative, then by every law, human and divine, we shall be justified in wresting it from Spain "
The document did little but claim the U.S. right to Cuba (by force if Spain refused to sell) and U.S. Southern cotton growers and sugar planters embraced it passionately, seeing in it a chance to extend slavery if Cuba became an American possession. Abolitionists and Northerners condemned the plan, and after the Black Warrior was released the excitement subsided.
Read the full text of the Ostend Manifesto