In early 1846 this being his third trip, Fremont showed up in Monterey with 60 armed men. This expedition was intended to be a scientific expedition, as his other past expeditions to the west had been. While in Monterey he went to see Thomas Larkin. Larkin a Monterey merchant (able to work in California because he would give the Dons credit) was working as a confidential agent for James Buchanan"s.

             Colonel Jose Castro took almost an instant dislike to Fremont. Castro ordered Fremont to leave California immediately. Fremont retreated to a nearby hill, the Hill of the Hawk and raised the American flag, although he apparently had no authorization to do anything of the sort from the U.S. government. Castro rides out to the Hill of the Hawk Larkin intervened to avoid violence, and Fremont rides north through great Sacramento Valley to southern Oregon the hole way killing Indians and destroying their villages. In and on the way to Sanoma is said Fremont started a rumor that Castro was going to expel all Americans from California. However the rumor started, it was soon believed throughout the mixed band of American settlers in the Sacramento and Napa valleys. .

             In Oregon, Fremont met Lieutenant Archibald Gillespie. While in Organ Fremont and his men get so drunk they were attacked by Indians while they were in a drunken slumber. Gillespie and Fremont decided to return to California together. It has been suggested that Gillespie brought secret orders from the secretary of the state to start an insurrection. He is said to have memorized and destroyed his orders. .

             With Fremont's "army" nearby, and the rumor that Castro was gathering forces to expel the Americans, the local settlers became emboldened enough to steal some horses meant for Castro's army, then to storm Vallejo's "fort" at Sonoma June 14, 1846. Vallejo's home had been built as a military outpost. Vallejo met the 30 armed men and invited three leaders, Ezekiel Merritt, William Knight, and Robert Semple, inside to discuss his surrender.