Little of Leontius' early life is known, other than that he was from
Isauria, and possibly of
Armenian descent.
Christian Settipani speculates that Leontius was the son of a certain Lazarus, who was the direct descendant of emperor
Phocas and general
Priscus. Furthermore, Settipani identifies
patrikios Tarasius as Leontius' son. Leontius was appointed as
strategos of the
Anatolic Theme, at the time the most senior military command of the Byzantine Empire, and
patrikios by Emperor
Constantine IV, possibly AD. Starting in 680, the Islamic
Umayyad Caliphate erupted into a civil war, known as the
Second Fitna. Umayyad authority was challenged even in their metropolitan province of
Syria, while most of the Caliphate recognized
Abdallah ibn al-Zubayr instead. Under
Marwan I and his son
Abd al-Malik, however, the Umayyads gained the upper hand, although the Zubayrids were not finally defeated until 692. The civil war in the Umayyad Caliphate provided an opportunity for the Byzantine Empire to attack its weakened rival, and, in 686, Emperor
Justinian II sent Leontius to invade Umayyad territory in
Armenia and
Iberia, where he campaigned successfully, before leading troops into
Media and
Caucasian Albania; during these campaigns he gathered loot. Leontius' successful campaigns compelled the Umayyad Caliph, Abd al-Malik ibn Marwan, to sue for peace in 688, agreeing to tender part of the taxes from Umayyad territory in Armenia,
Iberia, and
Cyprus, and to renew a treaty signed originally under Constantine IV, providing for a weekly tribute of 1,000 pieces of gold, one horse, and one slave. Justinian invaded the Caliphate again in 692, feeling that the Umayyads were in a weak position, but was repulsed at the
Battle of Sebastopolis, where a large number of
Slavs defected to the Umayyads, ensuring the Byzantine defeat. After this, the Umayyads renewed their invasion of
North Africa, aimed at taking the city of
Carthage in the
Exarchate of Africa, and also invaded
Anatolia. Around this time, Justinian imprisoned Leontius. Some Byzantine sources, such as
Nikephoros and
Theophanes, suggest that Justinian did so because he believed that Leontius was seeking to take the throne, but it is possible that the crushing defeat at Sebastopolis played a part in his imprisonment; as
strategos of the Anatolic Theme, he likely served in the battle, and may have even been the main Byzantine commander in it. After further setbacks in the war, Justinian released Leontius in 695 because he feared losing control of Carthage, and appointed him
strategos of the Theme of
Hellas in
Southern Greece. During his captivity, Leontius was cared for by two monks, Gregorios and Paulos, who prophesied his rise to the throne, and encouraged him to rise against Justinian after his release. Leontius, once free, quickly raised a rebellion against Justinian. Leontius had wide support from the aristocracy, who opposed Justinian's land policies, which restricted the aristocracy's ability to acquire land from peasant freeholders, and the peasantry, who opposed Justinian's tax policies, as well as the
Blue faction, and the
Patriarch of Constantinople Callinicus. Leontius and his supporters seized Justinian and brought him to the Hippodrome, where Justinian's
nose was cut off, a common
practice in Byzantine culture, done in order to remove threats to the throne, as mutilated people were traditionally barred from becoming emperor; however, Leontius did not kill Justinian, out of reverence for Constantine IV. After Justinian's nose was cut off, Leontius exiled him to
Cherson, a Byzantine
exclave in the
Crimea. ==Reign and downfall==