The Free Soil Party & the Ostend Manifesto
The Free Soil Party

There was growing conflict between pro slavery and antislavery forces in the U.S., and it was intensified when it gained new territories from Mexico and the continuing argument over whether or not slavery would be allowed in those territories. The defeat of the Wilmot Proviso, which tried to prevent the extension of slavery and the refusal of both the Whig and Democratic parties to endorse the principles of the proviso led the opposition groups to the need for a new party. The major groups involved in the organization of the Free-Soil party at a convention in Buffalo, N.Y., in 1848 included the abolitionist Liberty party, the antislavery Whigs, and the Barnburners, who were strongly opposed to slavery.The Free-soilers believed that all blacks should be excluded from the Mexican Cession. But contrary to abolitionists, they did not want the end of slavery. Instead they wanted to keep the West a land of opportunity for whites, so that they won't have to compete with the labor of slaves or free blacks. In 1848, the northerners that supported this group adopted "Free soil, free labor, and free men". They also wanted free homesteads(public land grants to small farmers) and internal improvements.
the campaign ad for Martin Van Buren and Charles Adams for the 1848 election

Their platform consisted of :

  • Opposition to the extension of slavery into the territories
  • Support for national internal improvement programs
  • Support for moderate tariffs designed for revenue only
  • Support for the enactment of a homestead law.

In the election of 1848, the Free-Soil Party nominated the former President Martin Van Buren.
The party polled 291,263 votes in the election of 1848; it carried no states, but it did turn the election in New York to the Whigs, while also playing a big role in the election of President Zachary Taylor. The party also elected 2 U.S. senators and 14 representatives. Most of its members then moved to the Republican Party.

The Ostend Manifesto
The southern expansionists were eager to obtain more territory, especially from Cuba. President Polk offered to buy Cuba from Spain for $100 million, but Spain refused to sell this remaining part of its empire. Several expansionists tried to take the island by force, but they were defeated, and those who participated were eventually executed by the Spanish. When Pierce was elected in 1852 as President, he had also adopted prosouthern policies and sent three American diplomats to Ostend, Belgium, where they secretly negotiated to buy Cuba from Spain. The Ostend Manifesto, made by James Buchanan, John Young Mason, and Pierre Soulé on Oct. 9, 1854, also insinuated that the US would use force, if necessary. It was later leaked to the press in the US and provoked an angry reaction from antislavery members of Congress, who accused him of trying to bring in a new slave state into the Union. President Pierce was forced to drop it.
President Pierce

Here's the full text of the Ostend Manifesto:

Sources: The APUSH textbook
AMSCO textbook