You can join the Stalkers right after you help the Stalkers drive the military out of the Cordon. You can join Duty, Freedom, and the Bandits after you flood the tunnels under the Agroprom Research Institute. To join a faction, you talk to the leader.
When you join a faction, you typically become an enemy of at least one other faction.
- If you join Freedom, Duty becomes
- If you join Stalkers,
Bandits become Hostile.
- If you join
Duty, Bandits and Freedom become
- If you join Bandits,
Stalkers and Duty become hostile,
Renegades become neutral.
You can switch factions that are still neutral or friendly to you after you join a faction. For example, if you initially joined the Bandits, you can leave and join Freedom. However, once a faction becomes hostile, you can't get them to become friendly with you again, even if you switch to another faction.
You can't get 'thrown out' of a faction, however, if you kill enough of your fellow faction members, your own faction will become hostile to you.
You lose some side-quests when joining a faction because some side-quests are from factions that become hostile.