Throughout the six episodes there are countless times where they mention ‘the war 3 years ago’ but no one actually explains or talks about what happened. So what do you all think happened?

I’m not too sure BUT I do think that when Mystery Badass Guy (I don’t know his name, the Psysician’s sidekick) came back from war and saw all his village burnt down, I think that village was the first zombie outbreak and instead of telling him what happened they just burnt the place down and left it untold and unexplained. We did see Lord Ahn Hyeon(?) pay his respects at the graves in one episode so I think he was the one who discovered and stopped the zombie villagers before the disease could spread. It would explain how he and his fighters knew how to kill the zombies in the episode they came to rescue everyone too.

What theories does everyone else have?

you are good!! i think u are right.......also 3 yrs ago was mentioned in kings palace too.  that villain guy asked physician lee to do what he did 3yrs ago to bring back king from dead.  so i think that doctor was involved in what happened 3 yrs ago in that village and that villain guy knows about it. villain also said  Lord Ahn Hyeon  would never go against may be he has something on him and  it is related to what happened in that village and thats why he never left mountains and as soon as he heard about disease he left mountains.