Question: The Second Civil War has begun. Ignoring how succession happens, the American west coast (including Baja California & the Mexican state of Sonora) becomes an independent republic. Their border runs from southern Sonora, runs along the eastern border Arizona & Utah, then finally up beside Idaho until it comes to Canada. 

Twenty-two years after the war began (this takes place in 2019), fighting has evolved into something like it has with North & South Korea, with nuclear weapons preventing the return of invasions or conventional warfare. For this border I described, what would the Western Republic need to monitor, patrol and protect this stretch of land? And with the Rocky Mountains serving as a natural barrier, how would this affect it?

Answer: 
When looking at a border of this size, there are multiple factors to consider. I'll use the US-Mexico border as an example, since it's almost the same size as yours.

**Technology**

Today at the US-Mexico border, there no single, continuous barrier (see [here](https://en.wikipedia.org/wiki/Mexico%E2%80%93United_States_border#Barrier)). Only about 25% is covered by the fence. In between the sections of fencing are varying forms of sensors. In your world, these could be cameras, seismic sensors, physical guard towers, border checkpoints, or moving patrols. 

Satellites would be impractical due to the number you would need to maintain a constant view of your border, but for the troop buildups along the border, like you describe. If US-191 is in our territory, it would become an equivalent to I-10(and US-90 east of Sierra Blanca, TX) on the US-Mexico, that is, a major artery for civilian traffic and border patrol. If not, then the same would apply to I-70, US-40, US-491, CO-90, and CO-141.

Depending on the military spending of the government, they would have Reapers and Global Hawks patrolling the border out of Peterson AFB(a decent choice, given it's on our side of the Rockies and the current home of [NORAD](https://en.wikipedia.org/wiki/North_American_Aerospace_Defense_Command))

**Geography**

A large section of the border is located in the center of the Rio Grande (see [here](https://en.wikipedia.org/wiki/Mexico%E2%80%93United_States_border#Border_region)). Parts of the Rio Grande are too fast moving to cross, so large parts of your border could be impassible due to geography. Of course this opens up opportunities for daring smuggler- or coyote-equivalents to try and find routes through (Hannibal and his *freaking war elephants* are a great example of the military using this tactic as well)
### If the border is in a natural place for a border then you shouldn't need to patrol it all that much.

Long term stable borders tend to be along mountain ranges, rivers, across seas and deserts etc. Places where an army naturally comes to a halt and moving further is a serious bit of logistics. The borders of Britain, Italy, and Spain for example.

On the other hand the borders of somewhere like France or Belgium have shown themselves not to be so stable over the past century or so. The [European plain](https://en.wikipedia.org/wiki/European_Plain) is a wonderful territory for marching an army and everyone from Napoleon to Hitler have tried it.

So if you've drawn your border down a mountain range with limited passes, then all you need to do is watch the passes and keep half an eye over the rest of the range but you don't need to man the whole thing.