The Republic of Baja California was proclaimed in 1853 by soldier of fortune William Walker.
He captured La Paz, declaring himself President of the Republic of Baja California.
Later he merged that into the Republic of Sonora.
On October 16, 1853, filibuster William Walker sailed out from San Francisco with 48 men on his first filibustering expedition: the conquest of the Mexican Baja California Territory and the state of Occidente. He financed the effort by selling scrip in exchange for promises of land in Sonora.
Three weeks later, Walker took control of La Paz, the capital of the sparsely populated Baja California, and 200 more men joined him. Walker declared La Paz the capital of a new "Republic of Baja California", with himself as president and a constitution copied from that of Louisiana.
Fearful of attacks by Mexico, Walker moved his position twice over the next three months, first to Cabo San Lucas, and then further north to Ensenada, Baja California. Here he abolished the "Republic of Baja California", instead pronouncing Baja California part of the larger Republic of Sonora.