Trans-Mississippi 
Mick Werve Santa Anna Rain Photography
As I see it, the Civil War was fought on four fronts:
Eastern Theater - Anything east of the Appalachian Mountains 
Western Theater - Everything West of the Appalachians to the Mississippi River
Trans-Mississippi Theater - The events west of the Mississippi
This page links to everything west of the Mississippi. I believe this was the theater where the South could have had the greatest impact on the war if they had tried harder. During the war most of the coverage was given to events on in the Eastern Theater. Large battle such as Shiloh and Chickamauga certainly made a splash in the eastern papers and the fall of Vicksburg was celebrated throughout the North, but battles like Wilson’s Creek, Pea Ridge and Valverde got much less coverage.
Neither side maintained large armies under uniform in most of the Trans-Mississippi arena; therefore much of the action was in the nature of guerrilla warfare and very ugly. People on both sides of the issue west of the Mississippi were caught between raiding bands of bushwhackers and redlegs, and never knew who was going demand their allegiance next.
In the far west the South made a half hearted attempt to secure an open port in California and capture the Santa Fe Trail to transport supplies to the South. Controling the Santa Fe Trail would also have allowed the South to control the overland gold shipments from the California gold fields. In the New MexicoTerritory there were uniformed soldiers fighting under commissioned officers unlike the savage gurrilar war fought in Kansas and Missouri. While Southern control of the gold from the California and the silver from Nevada would have been a serious boost to the Confederate war effort, the western Confederacy never got any real support in the way of resources from Richmond. Even without a serious commitment the southern forces under Brig. Gen. Sibley were able to capture all of New Mexico and most of Arizona, except Yuma. For a short time there was established the Confederate State of Arizona. Sibley carried orders from Jeff Davis to take California, Nevada and Colorado if possible and it is startling to think how close he came to pulling it off. There was even a small skirmish fought on the California border which technically the South won, it can only be speculated what might have happened if Richmond would have sent Silbley some soldiers and a few supplies.
There was a lot of support for "The Cause" in western mining camps. Unionville divided itself in two with Main Street as its own Masson-Dixon Line. A Southern sympathizer in Austin lost a bet and carried a 50 lbs sack of flour through town to the tune of the Battle Hyme of the Republic.