It consisted primarily of the three Confederate states west of the Mississippi (Texas, Louisiana, and Arkansas), the contested state of Missouri, and two Confederate territories - the Indian Territory and Confederate Arizona (roughly corresponding to the present-day states of Oklahoma, New Mexico, and Arizona).

[2] The Trans-Mississippi was the operational theater for many quasi-independent forces, including Quantrill's Raiders and the Missouri Bushwhackers.

The Army of the Trans-Mississippi[3] originally numbered well over 50,000 troops, but fewer than 43,000 were available by the end of the war.

General E. Kirby Smith, who commanded the Army, surrendered to Union forces on May 26, 1865, although by that point many of his troops had already deserted.

[5] The last remaining Army of the Trans-Mississippi force – and also the last remaining Confederate force – was the 1st Cherokee Mounted Rifles, commanded by Brigadier-General Stand Watie.