Risk-like Regions

I’m developing a game where regions as one might see in Risk, Empire Total War, or Victoria II, etc are a major aspect of gameplay, and I can’t seem to find a way to do them. I currently am trying to stay with using Unity’s built in terrain, but it seems that I can’t set up any sort of area that can detect clicks, etc and change color to display that it is selected. It seems that some sort of mesh is needed, but I’m a bit lost in how to make even that work. Any help is appreciated, thanks!

Easiest route to take is to make individual meshes with individual collision meshes for each region. Then you can detect clicks with raycasting and switch their material.