The slave owners need to annex Cuba as a slave state was becoming more pressing with increased abolitionist activity in England as well as New England. The British had already prohibited its merchants from engaging in the slave trade and were anxious to eradicate it elsewhere. Slave trade in the Caribbean had become their primary focus. If the British were successful, the American slave owners realized that slavery in the south would crumble. They foresaw that Africanization – supplanting slave labor with freed Africans – in Cuba would quickly spread to the United States. American slaves would have a safe haven just off shore to which they could escape.
Interestingly, Yankee businessmen and merchants seemed blind to the consequences of annexation of Cuba insofar as the issue of slavery was concerned. They were blinded by the commercial possibilities of free trade with Cuba, and helped fill Lopez's war chest as well.
All things considered, it appeared that Lopez's revolution was well on its way to success. However, when Lopez was finally ready to launch his first assaults in 1849, a new Administration came to power in the United States, and he faced the stubborn old warhorse, Zachary Taylor. Taylor announced that the neutrality laws against filibustering would be enforced.
Filibustering is derived from the Spanish filibustero, pirate. (In modern usage, to filibuster in the U.S. Senate is to pirate the argument and thus delay a vote.) Taylor sent the Navy to intercept Lopez's ships and arrest the rebels.
Taylor was dismayed to find himself assaulted on all sides. The American businessmen who financed Lopez complained vociferously and threatened to support anyone who would run against Taylor in the next election. The Spanish were not happy either. Even though Lopez was stopped and arrested, several times in fact, they complained bitterly when Southern courts repeatedly found him innocent and freed him to try again. It didn't help that a Mississippi governor accompanied Lopez on three occasions and was himself arrested, tried, and released.
The Lopez issue was ultimately resolved when someone alerted the Spanish army on Cuba, and Lopez was captured and garroted on the spot in a grisly public execution. His rebel force was summarily executed by firing squad.