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: 
If the purpose is to prevent infiltration, this is a solved problem. It actually is easier to implement the solution in gentle terrain than in rough terrain.

[Build a sturdy fence, topped with barbed wire, with a road / kill-zone behind it.](https://www.defenceprocurementinternational.com/features/land/middle-east-countries-use-blimps-and-sensors-to-beef-up-border-security) Optionally, build a second fence behind the road. The up-front cost is roughly 2 million dollars per mile, not including land costs.

Israel, Jordan, Saudi Arabia, Morocco, and Algeria have all built such fences.

The Great Wall of China performed the same purpose. It had a wall wide enough to be a road, with sheer drops on both sides. It made it difficult for small-scale raiders to return home with loot. Large-scale raiders bribed or coerced the border guards.
**Bullet trains.**

And by bullet I mean fast and also small. Existing rail lines are connected and upgraded, and armed trains zip up and down the line, looking for trouble at 250 mph. Maybe some of these are robot trains THE SIZE OF A VW bug; train drones. Bullet train drones that speak in train drone voices to anything that gets too close to the tracks!

Tracks are a good way to cross large distances and it is possible to use tracks to go really fast. And war trains are AWESOME!

[![WAR TRAINS](https://i.stack.imgur.com/VQ0SS.jpg)](https://i.stack.imgur.com/VQ0SS.jpg)
<http://www.darkroastedblend.com/2013/05/awesome-armoured-trains-and-rail.html>