<P> Although they were illegal, filibustering operations in the late 1840s and early 1850s were romanticized in the United States . The Democratic Party's national platform included a plank that specifically endorsed William Walker's filibustering in Nicaragua . Wealthy American expansionists financed dozens of expeditions, usually based out of New Orleans, New York, and San Francisco . The primary target of manifest destiny's filibusters was Latin America but there were isolated incidents elsewhere . Mexico was a favorite target of organizations devoted to filibustering, like the Knights of the Golden Circle . William Walker got his start as a filibuster in an ill - advised attempt to separate the Mexican states Sonora and Baja California . Narciso López, a near second in fame and success, spent his efforts trying to secure Cuba from the Spanish Empire . </P> <P> The United States had long been interested in acquiring Cuba from the declining Spanish Empire . As with Texas, Oregon, and California, American policy makers were concerned that Cuba would fall into British hands, which, according to the thinking of the Monroe Doctrine, would constitute a threat to the interests of the United States . Prompted by John L. O'Sullivan, in 1848 President Polk offered to buy Cuba from Spain for $100 million . Polk feared that filibustering would hurt his effort to buy the island, and so he informed the Spanish of an attempt by the Cuban filibuster Narciso López to seize Cuba by force and annex it to the United States, foiling the plot . Nevertheless, Spain declined to sell the island, which ended Polk's efforts to acquire Cuba . O'Sullivan, on the other hand eventually landed in legal trouble . </P> <P> Filibustering continued to be a major concern for presidents after Polk . Whigs presidents Zachary Taylor and Millard Fillmore tried to suppress the expeditions . When the Democrats recaptured the White House in 1852 with the election of Franklin Pierce, a filibustering effort by John A. Quitman to acquire Cuba received the tentative support of the president . Pierce backed off, however, and instead renewed the offer to buy the island, this time for $130 million . When the public learned of the Ostend Manifesto in 1854, which argued that the United States could seize Cuba by force if Spain refused to sell, this effectively killed the effort to acquire the island . The public now linked expansion with slavery; if manifest destiny had once enjoyed widespread popular approval, this was no longer true . </P> <P> Filibusters like William Walker continued to garner headlines in the late 1850s, but to little effect . Expansionism was among the various issues that played a role in the coming of the war . With the divisive question of the expansion of slavery, Northerners and Southerners, in effect, were coming to define manifest destiny in different ways, undermining nationalism as a unifying force . According to Frederick Merk, "The doctrine of Manifest Destiny, which in the 1840s had seemed Heaven - sent, proved to have been a bomb wrapped up in idealism ." </P>

The belief that the united states had the right and duty