The Western Roman Empire legions won some great victories but it was internal political instability which ultimately killed it

As for the Eastern Roman Empire it great political positioning joining the Eastern world with the Western world which also enabled

The Empire to get rich due to access to the silk road

The Eastern Roman empires long decline began when it lost Egypt in the seventh century due to the rise of Islam

It also led to the slow decline of the city of Constantinople as well

At the height of irony the ottoman Empire and the Turkish state suffered the same slow decay as the Eastern Roman Empire