Proponents of manifest destiny did not stop with the end of the Mexican War and the Mexican Cessions. The fierce debates and the radicals on both sides of the slavery issue were loudly calling for action and the voices of compromise were few and far between. The spread of American virtues as part of an ongoing mission could no longer be the role of government as it struggled to hold the Union together during the 1850s. Many of the more ardent believers in the concept of manifest destiny took matters into their own hands. They used the technique of military filibustering.
Before filibuster meant the hijacking of the US Congress by holding the floor indefinitely, it was a term applied to militants who acted without approval or support of legally constituted authority (although at times that authority was a covert partner). One example was William Walker, who eyed the Mexican peninsula of Baja California and decided under manifest destiny it was his duty to spread American virtue and the blessings of liberty to the populace there. He found the populace less than enthused about the idea, and his attempt failed. After returning to California, where he was tried for inciting a war and acquitted in eight minutes, he decided his duty lay in Nicaragua.
Nicaragua was a major trade route between California and the east, as it offered a passage by water across the southern portion of the country. In 1854 Nicaragua was involved in a civil war between Conservatives and Democrats, and Walker obtained a contract to serve as a mercenary for the Democrats. He landed in Nicaragua with sixty men he had recruited. Supported by Democratic troops, Walker defeated the Conservatives and established himself as de facto ruler of Nicaragua, obtaining recognition from US President Franklin Pierce in 1856. He then nationalized the steamboat lines owned by Cornelius Vanderbilt, creating a powerful enemy.
Vanderbilt dispatched agents to regain control of his property, which provided a supply line for Walker’s troops. Meanwhile Walker established American virtue and liberty by holding a rigged election in which he became President of Nicaragua. He then began to court immigration and support from the Southern United States, reinstituted slavery, declared English to be the official language, and introduced a new monetary system. The Nicaraguan experiment was opposed by its neighbors and Walker was eventually defeated in the war he had started, and surrendered to the US Navy.
Repatriated to New York, he was soon off on another attempt to spread manifest destiny to Central America, this in Honduras. After misadventures there he was executed by the government of Honduras. There were many other filibusters before the American Civil War, including attempts to carry American manifest destiny to Cuba, Canada, and in the countries of Central America, causing President Millard Fillmore to address the issue in his annual message to Congress, now referred to as the State of the Union Address. Fillmore spent more time discussing filibustering than he did the rising threats of secession in the South.