Like I already played Battlefield 2, I want something like that 🥺
There is Syrian Warfare. https://store.steampowered.com/app/485980/Syrian_Warfare/
There is also Gesta Final which is a Cuban video game, you will have to pirate it and maybe it will be easier to run it with Proton than with Windows.
https://www.ecured.cu/Gesta_Final
And there is also a Famicom Japanese game called Gebara where you play as el Che and Castro, but you need to play the Japanese version since the Usonian was censored and they changed the story.
Edit: I just noticed you said FPS, but well, hope you like them anyway
Does Red Faction count?
not exactly militia, but you can play the best army on enlisted, make that ppsh go brrrrrrrrrrrr
Yea! Doming nazis in enlisted, fun!
squad allows you to play as local insurgent militias against the US or UK in a very tactical setting with a huge map. you can strap IEDs to vehicles and do all sorts of cool stuff. the game also allows you to play as the PLA and PLA marines sometimes, which is also super cool.
homefront revolution
Good ol’ Rising Storm 2
Insurgency is a fun multiplayer one, I haven’t played the newer version a ton but it feels like a bit more realistic battlefield while being a bit more arcade-y than other sim type FPS.
it’s probably not what you’re looking for but Thatcher’s Techbase
Arma 3, with mods you can make any scenario you want pretty much and there are a number of scenarios made by community which center around guerrilla warfare.