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The idea of additional territorial expansion remained popular with the U.S. [ public, as did the idea of spreading republican government. These expansionist dreams were aided at first by a Venezuelan-born resident of Cuba, Narciso López, who, like some wealthy Cuban slave-owners, was wary of shaky Spanish rule over the island, and thus sought to have it annexed by the United States in order to ensure slavery’s preservation in Cuba. Despite this official ambivalence toward these filibusterers, U.S. leaders often protested the lack of due process by which foreign governments imprisoned and executed U.S. citizens involved in filibustering. Overall, filibustering and U.S. policies seem to have hindered more than helped U.S. attempts at territorial expansion in this time period. The Gadsden Purchase was the only official territorial acquisition during the 1850s. Filibustering tended to encourage local hostility to U.S. expansion and spread international resistance to growing U.S. power. U.S. expansion in the later 1850s was also hindered by domestic sectional tensions over slavery.
Expansion towards the South was favored by the slave-owning aristocracy of the South, who feared that heavy immigration to the North, and expansion into territory north of the Missouri Compromise line would, sooner or later, tip the balance of power in the Federal government away from the South and put the peculiar institution at risk. Additions to the South were free of the Missouri Compromise problem and could perhaps be induced to become slave states, as the example of Texas showed, despite the fact that slavery was illegal in Spanish possessions and former colonies at the time.
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