Fremont and Carson set out on their third exploratory expedition. Although Fremont claimed that his journeys were strictly scientific, they had political motivations. “Overtly or not,” Sides argues, “his larger purpose was to advance the cause of American emigration, American expansion, [and] American hemispheric hegemony” (110). This time, Fremont and Carson’s official mandate was to map the eastern slope of the Rocky Mountains, but Fremont seemed to have been operating under “secret” orders from the highest levels of government which would take them further west, to the Mexican province of California.

By the beginning of the expedition in 1845, California was only loosely tied to Mexico. It had already undergone several revolutions and was steadily becoming Americanized. By Winter 1846, Fremont was in California promising protection to American settlers there, should war (inevitably) break out. The Mexican government justifiably took issue with their presence, and Fremont responded with “pure histrionics,” building a fort at Gavilan Peak—the “un-Alamo”—at which he loudly proclaimed himself willing to die.

Convinced by Carson and his men to back down, Fremont still lingered in nearby Oregon. “He seemed to be stalling for time,” Sides writes, “hovering within striking distance, waiting for something to break” (113-4). This “break” came in the form of the courier Lieutenant Archibald Gillespie, who arrived from Washington with an extremely sensitive dispatch to Fremont, likely from President Polk.