William Walker (May 8, 1824 – September 12, 1860) was an American 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.