They may take the country just north of Persia when they arrive but when they attack Persia it self the kingdom of Persia in ai hands more often than not crushes the invaders. The invaders should be given more troops to make sure they crush Persia even if its controlled by the player. They even have trouble with local pagans if they ever get organized in that area. They do not even make it to roman empire to attack it like they did in history.