U.S. Brigadier General Stephen Watts Kearney, having departed in July 1846 immediately upon receiving news of the war with Mexico, leads the seventeen hundred Missouri volunteers of the grandly titled Army of the West from Fort Leavenworth, Kansas, down the Santa Fe Trail into New Mexico.

Kearny's orders are to secure the territories Nuevo México and Alta California.

In Santa Fe, Governor Manuel Armijo wants to avoid battle, but on August 9, Catholic priests, Diego Archuleta (the young regular-army commander), and the young militia officers Manuel Chaves and Miguel Pino force him to muster a defense.