Although the Byzantines got off the wrong foot, what with the invasion of the first Caliphate, it managed to survive until the 14th century by pure wits. It managed to defend itself from the Caliphates, the Seljuks, and as I would like to argue, even the Crusaders.
The Downfall of the Byzantines came with the fourth crusade, and although the Byzantine Empire reformed, it was well past its prime.
