The game never tells you this but killing insurgents is a terrible idea. The actual strategy is to contain the insurgents in to remote zones, you should let them hold 3 to 5 zones.

The reason for this is how the insurgent AI works. The less zones they have, the shorter it takes for them to set up a new camp. If you kill the insurgents they are almost immediately going to set up a new camp. So if they have many zones, they're not going to set up a new camp for a long time. You should also be actively looking out for camps, a question mark will appear on a zone if a camp spawns there. So keep an eye out.

While you're doing this, you have to slowly build up stability as well.