The map of GTA 5 is very big, and the military base (Fort Zancudo) is not clearly given on the map, if you are a new gamer, then it is not easy to find the military base. So, I have located a military base on the map of GTA 5 ( Online + Offline/story mode) which you can see below. After seeing the location, you can go to a military base in your own GTA 5 game.
The military base of GTA 5 is also called Fort Zancudo, there is only one military base in GTA 5 which is exciting and dangerous. It is located near Mount Josiah mountain.
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It is very tough to survive in a military base once you go inside the military base because no other people are allowed, even if you cross the military base via helicopter, then very dangerous wanted level rises & military man damage or blast your helicopter or airplane by jet fighter planes.