Trans-Mississippi Theater of the American Civil War

The Trans-Mississippi Theater of the American Civil War consists 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.

The campaign classification established by the United States National Park Service 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 Arkansas in March 1862, solidifying Union control over most of Missouri. The areas of Missouri, Kansas, and Indian Territory (modern-day Oklahoma) were marked by extensive guerrilla activity throughout the rest of the war, the most famous incident being the Lawrence massacre of 1863.

In 1862, Confederate forces pushed north through the New Mexico Territory, but were stopped at the Battle of Glorieta Pass. In 1863, General Edmund Kirby Smith took command of the Confederate Trans-Mississippi Department, and unsuccessfully tried to relieve the Siege of Vicksburg. As a result of the siege, the Union gained control of the entire Mississippi River. This left the Trans-Mississippi Department almost completely isolated from the rest of the Confederacy. It became known as "Kirby Smithdom", emphasizing the Confederate Government's lack of control over the region.

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