How to join the military?

Is it possible to join the military in GTA 5? If so, how?

I think you greatly misunderstand what kind of game GTA is. It is not a game where you "join" anything or anyone; it is a game where your player character lives his life by completing missions for (sometimes random) individuals.

The only GTA game I know of, in which you are actually part of the military, is Vice City Stories in which you start as a corporal doing odd jobs for your corrupt superior, which eventually leads to your discharge from the army fairly early on.

Although you can play as a firefighter or policeman in most GTA games, you are never actually a real firefighter or policeman, only a random criminal who stole a firetruck or cop car and does their job for fun.

In GTA 5 you can...

• pilot fighter aircrafts or other military planes, assuming you manage to steal one from Fort Zancudo.
• pretend to be a soldier or marine if you've got the right clothes and weapons. Soldiers will still try to kill you if you enter Fort Zancudo, since you're never actually a real soldier.
• pilot helicopters, even military ones. Again, this is assuming you manage to steal them first. The Buzzard can be bought as well.
• skydive. Whenever you enter an aircraft, you'll automatically get a parachute.
• pilot a tank or other military vehicles.

If you want to be a military police officer, general, or whatever military worker you fancy, you'll have to do with make believe.

You can, in a roleplay server on FiveM (a multiplayer mod for GTA V).

If you are playing on PC or a j-tagged XBOX or PlayStation on can install the Police Mod. It is free and requires Script Hook V. It is downloadable from https://www.gta5-mods.com/scripts/police-mod-1-0b. Remember as you need Script Hook V, you will not be able to go online for the duration in which you have these mods.