 Mustafa was taken and put to death by the sultan, who then turned his arms against the Roman emperor and declared his resolution to punish the Palaiologos for their unprovoked enmity by the capture of Constantinople.Murad II then formed a new army called Azap in 1421 and marched through the Byzantine Empire and laid siege to Constantinople.While Murad was besieging the city, the Byzantines, in league with some independent Turkish Anatolian states, sent the sultan's younger brother Küçük Mustafa (who was only 13 years old) to rebel against the sultan and besiege Bursa.