In March Sibley sent a Confederate force of 200-300 Texans under the command of Maj. Charles L. Pyron on an advance expedition over the Glorieta Pass, a strategic location on the Santa Fe Trail at the southern tip of the Sangre de Cristo Mountains southeast of Santa Fe. Control of the pass would allow the Confederates to advance onto the High Plains and make an assault on Fort Union, a Union stronghold on the route northward over Raton Pass. Sibley sent six companies under the command of Col. Tom Green to block the eastern end of Glorieta Pass, turning any Union defensive position in the Sangre de Cristos.