American frontier
Trans-Mississippi theater

The Confederacy engaged in several important campaigns 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 lines enabled pro-Confederate guerrillas, such as Quantrill's Raiders, to attack Union strongholds and massacre the residents.

In Texas, citizens voted to join the Confederacy; anti-war Germans were hanged. Local troops took over the federal arsenal in San Antonio, with plans to grab the territories of northern New Mexico, Utah, and Colorado, and possibly California. Confederate Arizona was created by Arizona citizens who wanted protection against Apache raids after the United States Army units were moved out. The Confederacy then sets its sight to gain control of the New Mexico Territory. General Henry Hopkins Sibley was tasked for the campaign, and together with his New Mexico Army, marched right up the Rio Grande in an attempt to take the mineral wealth of Colorado as well as California. The First Regiment of Volunteers discovered the rebels, and they immediately warned and joined the Yankees at Fort Union. The Battle of Glorieta Pass soon erupted, and the Union ended the Confederate campaign and the area west of Texas remained in Union hands.

Missouri, a Border South 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; he was aided by Confederate forces from Arkansas and Louisiana. The Governor of Missouri and part of the state legislature signed an Ordinance of Secession at Neosho, forming the Confederate government of Missouri, and the Confederacy controlling Southern Missouri. However, Union General Samuel Curtis regained St. Louis and all of Missouri for the Union. The state was the scene of numerous raids and guerrilla warfare in the west.
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