Trans-Mississippi theater of the American Civil War

The trans-Mississippi theater of the American Civil War was the scene of the major military operations west of the Mississippi River. The area is often thought of as excluding the states and territories bordering the Pacific Ocean, which formed the Pacific coast theater of the American Civil War (1861–1865).

The campaign classification established by the National Park Service of the U.S. Department of the Interior^[1] is more fine-grained than the one used in this article. Some minor NPS campaigns have been omitted and some have been combined into larger categories. Only a few of the 75 major battles the NPS classifies for this theater are described. Boxed text in the right margin show the NPS campaigns associated with each section.

Activity in this theater in 1861 was dominated largely by the dispute over the status of the border state of Missouri. The Missouri State Guard, allied with the Confederacy, won important victories at the Battle of Wilson's Creek and the First Battle of Lexington. However, they were driven back at the First Battle of Springfield. A Union army under Samuel Ryan Curtis defeated the Confederate forces at the Battle of Pea Ridge in northwest Arkansas in March 1862, solidifying Union control over most of Missouri. The areas of Missouri, Kansas, and the Indian Territory (modern-day Oklahoma) were marked by extensive guerrilla activity throughout the rest of the war, the most well-known incident being the infamous Lawrence massacre in the Unionist town of Lawrence, Kansas of August 1863.

In the spring of 1862, Confederate forces under Henry H. Sibley pushed north along the Rio Grande from El Paso, Texas into New Mexico Territory, but despite their initial success at the Battle of Valverde, were stopped at the Battle of Glorieta Pass (March 26–28, 1862). In 1863, General Edmund Kirby Smith took command of the Confederate Trans-Mississippi Department, and unsuccessfully tried to relieve the siege of Vicksburg by Lt. Gen. Ulysses S. Grant on the opposite eastern banks of the Mississippi River in the state of Mississippi. As a result of the long campaign, siege, and surrender in July 1863 by Gen. John C. Pemberton, the Union gained control of the entire Mississippi River, splitting the Confederacy. This left the Trans-Mississippi Department almost completely isolated from the rest of the Confederate States to the east. It became nicknamed and known as "Kirby Smithdom", emphasizing the Confederate Government's lack of direct control over the region.

In the 1864 Red River Campaign, a U.S. force under Major General Nathaniel P. Banks tried to gain control over northwestern Louisiana, but was thwarted by Confederate troops commanded by Richard Taylor. Price's Raid, an attempt led by Major General Sterling Price to recapture Missouri for the Confederacy, ended when Price's troops were defeated in the Battle of Westport that October. On June 2, 1865, after all other major Confederate armies in the field to the east had surrendered, Kirby Smith officially surrendered his command in Galveston, Texas. On June 23, Stand Watie, who commanded Southern troops in the Indian Territory, became the last Confederate general to surrender.