[2] Democratic Senator Thomas Hart Benton of Missouri was a prominent leader of this movement, into which he enlisted his son-in-law, John C. Frémont.
In 1845, Captain Frémont was sent by the War Department on an expedition to survey the Great Basin and Alta California, a possession of Mexico.
[3][4] Upon arriving in California, Frémont and his men moved about the northern half of the state for several months, provoking the Mexican authorities and building up patriotic sentiment among Americans who had settled there.
A message from the American government implying war with Mexico was imminent led Frémont and his men to return to California in late May.
[8] Frémont's band then proceeded to other rancherias and attempted to attack them as well, but word had spread and the Indians across the land were already fleeing.
John C. Frémont became Military Governor of California in January 1847, but was forced to give up the position less than two months later under disputed circumstances.