Ha, ha,
You dont know what you are talking about. Bulgaria has helped Byzantium against the invasions of the Ottomans.The problems was that Byzantium wanted to conquer Bulgaria and bulgarians just couldnt give in without a fight
The reason the Ottomans conquered Byzantium and the Bulgarians is the fourth crusade which killed and set on fire the balkan ortodox christians teritories. After this all these nations weakened and were soon easily conquered by the Ottomans.
The Pope is still apologizing what the catholic church did.