Why do they attack, we are fellow Romans?

I’ve finally turned a profit playing as the Western Empire Romans in BI. You would think that I would be happy with that, well I’m not at about turn 10 every time the eastern empire attacks me and take’s my cities. Why, why would they do that we are allies why do they attack me.
:furious3:

Because they want to rule the whole empire, not just the east.

To win the Game, ERE must control Rome.

Because the ERE requires Rome and Carthage to win, so you're definitely going to get attacked by them. Also, why not go for the usually bankrupt and weak WRE instead of facing those scary Sasanid Immortals?