Theodosius III resigns the throne to the Byzantine Empire to enter the clergy allowing Leo III to take the throne and begin the Isaurian dynasty.

Theodosius III did not fight for the throne and did not fight to keep it. The Byzantine emperor, who had held power for barely two years, resigned on March 25, 717, and entered a monastery, allowing Leo III to take the crown and establish the Isaurian dynasty. It was one of the few peaceful transfers of power in Byzantine history, a system that more typically resolved succession disputes through blinding, poisoning, or battlefield slaughter. Theodosius had never wanted to be emperor. He was a tax collector from Adramyttium who was proclaimed emperor by the Opsikion army in 715 during a period of military revolt against Emperor Anastasius II. The troops essentially forced the reluctant bureaucrat onto the throne after Anastasius failed to defend against Arab raids. Theodosius had no military experience, no political base, and no appetite for the violence required to maintain power in Constantinople. Leo, the strategos (military governor) of the Anatolikon theme, was everything Theodosius was not: a career soldier from the empire's Syrian frontier who understood both Arab military tactics and Byzantine court politics. When Leo marched on Constantinople with his army, Theodosius negotiated rather than fought, receiving guarantees of personal safety for himself and his son in exchange for abdication. Both entered religious life, and Leo was crowned without bloodshed. The timing was critical. Within months of taking power, Leo III faced the massive Arab siege of Constantinople in 717-718, one of the most important military engagements in medieval history. His defense of the city using Greek fire, strategic alliances with the Bulgars, and exploitation of a harsh winter saved the Byzantine Empire and, many historians argue, prevented the Islamic conquest of southeastern Europe. Theodosius's quiet departure put the right general in power at the exact moment the empire needed one.