That doesn't work from what I remember, it gets overwritten eventually since one faction usually makes peace. You would have to edit the diplomacy and truce system, which is probably not feasible or possible in .txt alone.
I've seen a few mods implement this, such as ACOK and AWOIAF, so how would one make a locked war between two factions that never ends until the other faction is wiped out?
I'm wanting to add it to gabrilduro's old mods, American Revolution and Civil War, so that the titular wars actually occur.