William Walker (May 8, 1824 – September 12, 1860) was an American physician, lawyer, journalist, and mercenary. In the era of the expansion of the United States, driven by the doctrine of "manifest destiny", Walker organized unauthorized military expeditions into Mexico and Central America with the intention of establishing colonies. Such an enterprise was known at the time as "filibustering". After settling in California, motivated by an earlier filibustering project of Gaston de Raousset-Boulbon, Walker attempted in 1853–54 to take Baja California and Sonora. He declared those territories to be an independent Republic of Sonora, but he was soon driven back to California by the Mexican forces. Walker then went to Nicaragua in 1855 as leader of a mercenary army employed by the Nicaraguan Democratic Party in its civil war against the Legitimists. He took control of the Nicaraguan government and in July 1856 set himself up as the country's president.[1] Walker's regime was recognized as the legitimate government of Nicaragua by US President Franklin Pierce, and it initially enjoyed the support of some important sectors within Nicaraguan society.[2] However, Walker antagonized the powerful Wall Street tycoon Cornelius Vanderbilt by expropriating Vanderbilt's Accessory Transit Company, which operated one of the main routes for the transport of passengers going from New York City to San Francisco. The British Empire saw Walker as a threat to its interests in the possible construction of a Nicaragua Canal. As ruler of Nicaragua, Walker re-legalized slavery, although this measure was never enforced, and threatened the independence of neighboring Central American republics. A military coalition led by Costa Rica defeated Walker and forced him to resign the presidency of Nicaragua on May 1, 1857.[3] Walker tried to re-launch his filibustering project and sought renewed support from pro-slavery forces in the Southern United States on the eve of the American Civil War. In 1860, he published a book titled The War in Nicaragua, which promoted his efforts to conquer Central America in order to expand slavery geographically. That year, he returned to Central America, where the Royal Navy arrested him and handed him to the government of Honduras, which executed him.