How to easily win as the Eastern Roman empire in two simple steps

Step one: aggressively invade the western roman empire as soon as you kill the visigoths

Step two: become friends with all the barbarians and trade as much as possible

why?: the western roman empire is simply unable to stop your steamroll advance and all the factions that would of normally invaded you become your friend due to your violence commited against the western romans (also all the trade with the barbarians gives you loads of money)

notes: once all the western romans are dead the barbarians will slowly start to like you less.
to prevent having to deal with the fallout of this simply change corse and start beating up the huns instead. by the time attilas gone you will be unbeatable