If you are referring to the Sibley Campaign, then El Paso is the CSA supply base with an Arsenal, a Powder Mill, etc. Create the Partisan unit in Western AZ from the Regional Decision (RGD) card and send it to NM set to Green/Green Evade Combat. As soon as your group with Canby unlocks, blow the depot at Fort Craig and fall back to Albuquerque. Now for the CSA to advance any further than Ft. Craig, they will have to build a depot there from a Supply wagon. The Partisan unit has two RGD cards that allow it to destroy an enemy depot. Sibley starts with 2 Supply Units (SU) and can build one more. So Sibley builds the Ft. Craig depot and the Partisan eventually destroys it twice, leaving him with only the one SU for his stack until he can afford to build more. The Partisan unit has two other RGDs that weaken 15 elements of the CSA Division by lowering cohesion, setting it up for extinction. Also, several of the RGDs provide VPs, so make use of those too. Meanwhile, the Union has more resources and can eventually overwhelm the CSA to take El Paso.
I'm the 51st shade of gray. Eat, pray, Charge!