[1] 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.
[2][3] Upon arriving in western Alta California, Frémont and his men moved about the northern half of the present-day state for several months, provoking the Mexican authorities and building upon grievances and patriotic sentiment among Americans who had settled there.
The party reached Reading's Ranch (near present-day Redding, California) on 5 April 1846 and spotted a large native camp.
The soldiers advanced from three sides on the Wintu, who were unable to flee the camp as the majority of their ranks were women and children and they were pinned against the river.
Breckenridge writes: The settlers charged into the village taking the warriors by surprise and then commenced a scene of slaughter which is unequalled in the West.
He quickly returned from Oregon to participate, killing several more Sacramento Valley Native Americans in the journey south in the Sutter Buttes massacre.
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.
To some degree this saved the local Wintu from immediate annihilation, though smaller scale massacres occurred as early as the next year.