Squad Reserves in Just Cause 4 are required for securing regions. Without them you will not be able to take the location from your opponent and thus unlock subsequent quests of the main thread. The process of securing regions is described in the "Securing Regions" section of our guide; this page answers the question of how to acquire Squad Reserves in Just Cause 4.
Increasing the level of Chaos
The main way to earn additional Squad Reserves is to increase the level of Chaos. Its bar is located in the top right corner of the screen and is increased each time you destroy objects in enemy posts. Destroying turrets, generators or fuel tanks will increase the level of Chaos, and if the bar fills up to the end, you will receive several Squad Reserves. Primarily look for red objects. Destroying your opponent's vehicles is also worth it, though.
Squad Reserves are needed to Secure Regions, but in some cases, it is the securing of regions that provides additional reserves. This happens when the region you are taking over borders with at least two of your regions (i.e. those you have already taken over). Then, instead of spending a branch reserve, you will receive an additional amount of it.