From Wikipedia

The West
Episode Four
(1856 - 1868).

Death Runs Riot

At the outset of the American Civil War, Westerners looked to the Civil War to settle the question of slavery in their territories. But they also feared that the federal government would be too preoccupied with the war to worry about the stability of the territorial governments and that lawlessness might spread. The Dred Scott Decision had made the choice of slavery legal in all of the land west of the Mississippi River, except for Kansas, Oregon, and California.

Although most of the battles of the Civil War took place east of the Mississippi River, a few important campaigns occurred in the West. However, Kansas, a major area of conflict building up to the war, was the scene of only one battle, at Mine Creek. But its proximity to Confederate states enabled guerillas, such as Quantrill's Raiders, to attack Union strongholds, causing considerable damage. Both sides attacked civilians, murdering and plundering with little discrimination, creating an atmosphere of terror.

In Texas, citizens voted to join the confederacy. Local troops took over the federal arsenal in San Antonio, with plans to grab the territories of New Mexico, Utah, and Colorado, and possibly California. At the Battle of Glorieta Pass, the Texans' campaign was defeated by Union troops from Colorado and from Fort Union. Missouri, a Union state where slavery was legal, became a battleground when the pro-secession governor, against the vote of the legislature, led troops to the federal arsenal at St. Louis. When Confederate forces from Arkansas and Louisiana joined him, Union General Samuel Curtis was dispatched to the area and regained Missouri for the Union for the duration of the war.

The decreased presence of Union troops in the West left behind untrained militias which encouraged native uprisings and skirmishes with settlers. President Lincoln appears to have had little time to formulate new Indian policy. Some tribes took sides in the war, even forming regiments that joined the Union or the rebel cause, while others took the opportunity to avenge past wrongs by the federal government. Engagements were fought against Indians in Utah (Shoshones), Colorado (Apaches), and New Mexico (Navajo). Within the "Indian Territory" (later Oklahoma), conflicts arose among the Five Civilized Tribes, some of whom sided with the South being slaveholders themselves.

The West