In the Trans-Mississippi Theater, most of the fighting took place in Missouri between the pro-secessionist Missouri State Guard, commanded by Major General Sterling Price, and the Union Department of the West. Union forces under the command of Brigadier General Nathaniel Lyon drove the Missouri State Guard and the pro-secessionist portion of the state government into the southwestern part of the state, where it united with the Confederate Western Army commanded by Brigadier General Benjamin McCulloch. There, on August 10 in the Battle of Wilson's Creek, Lyon attacked the combined forces of Price and McCulloch and was defeated, with Lyon being killed during the battle. McCulloch returned to Arkansas, while Price moved north, attempting to recapture the state from Union forces.Foote, pp. 90–95, 98. Another Confederate offensive took place in the New Mexico Territory, where a Confederate cavalry battalion moved into the southwestern part of the territory and captured Fort Filmore, forcing the surrender of the Union garrison. A secessionist convention in Mesilla declared the southern portion of the territory to be the Confederate Territory of Arizona and raised several militia companies, which fought several skirmishes with both the Union forces remaining in the territory and the Apache tribes.Josephy, pp. 44–51.
