Editing Theodosius III (section)

===Background===
[[File:ByzantineEmpire717+extrainfo+themes.svg|thumb|left|upright=1.5|A map of the Byzantine Empire during the 20 Years' Anarchy|alt=A colored map showing the extent of the Byzantine Empire in 717]]
After the [[Umayyad Caliphate]] was repelled at the [[Siege of Constantinople (674–678)|first Arab siege of Constantinople]] (674–678), the Arabs and [[Byzantine Empire|Byzantines]] experienced a period of peace between each other.{{sfn|Lilie|1976|pp=81–82, 97–106}} Hostilities were resumed by Byzantine Emperor [[Justinian II]] ({{reign|685|695|705|711}}), resulting in a string of Arab victories. As a consequence, the Byzantines lost control over [[Armenia (region)|Armenia]] and the [[Caucasus|Caucasian]] principalities, and the Arabs gradually encroached upon the Byzantine borderlands. Annually, generals from the Caliphate would launch raids into Byzantine territory, seizing fortresses and towns.{{sfn|Blankinship|1994|p=31}}{{sfn|Haldon|1990|p=72}}{{sfn|Lilie|1976|pp=107–120}} After 712, the defenses of the Byzantine Empire weakened, as Arab raids penetrated deeper into Byzantine [[Asia Minor]], and Byzantine response to these raids became less common; much of the frontier became depopulated, as the inhabitants were either killed, enslaved, or driven away, a result of this was that many frontier forts, especially in [[Cilicia]], were gradually abandoned.{{sfn|Haldon|1990|p=80}}{{sfn|Lilie|1976|pp=120–122, 139–140}} The success of these raids emboldened the Arabs, who prepared for a second assault against [[Constantinople]] as early as the reign of Caliph [[al-Walid I]] ({{reign|705|715}}). After his death, his successor, [[Sulayman ibn Abd al-Malik|Sulayman]] ({{reign|715|717}}) continued planning the campaign,{{sfn|Guilland|1955|p=110}}{{sfn|Lilie|1976|p=122}}{{sfn|Treadgold|1997|p=344}} Sulayman began assembling his forces in late 716, on the plain of [[Dabiq, Syria|Dabiq]], north of [[Aleppo]], entrusting the command of these forces to his brother, [[Maslama ibn Abd al-Malik]].{{sfn|Guilland|1955|pp=110–111}}{{sfn|Eisener|1997|p=821}}

The [[Early Slavs|Slavs]] and [[Bulgars]] also formed a growing threat to the northern frontier of the Byzantine Empire, threatening Byzantine control in [[the Balkans]].{{sfn|Vasiliev|1980|p=229}} During the rule of Byzantine Emperor [[Philippicus]] ({{reign|711|713}}), in 712, the Bulgarians under Khan [[Tervel]] advanced as far as the walls of Constantinople itself, plundering the surrounding country, including villas and estates near the capital, where the Byzantine elites often summered.{{sfn|Ostrogorsky|1957|p=136}}

Theodosius came to power during a period called the [[Twenty Years' Anarchy]], defined by struggles between the emperors and the elites, and political instability, with a rapid succession of emperors. The nobles of this time were often natives of Asia Minor, and rarely had a strong agenda beyond preventing the emperors from growing stronger and disrupting the status quo.{{sfn|Bury|1889|pp=384–385}}{{sfn|Jenkins|1987|p=60}} The Twenty Years' Anarchy began when Emperor Justinian II was overthrown by [[Leontius]] ({{reign|695|698}}) in 695, ending the [[Byzantine Empire under the Heraclian dynasty|Heraclian dynasty]], which had retained power for eighty years. During this period of anarchy, seven different emperors took the throne, including a restored Justinian for a time.{{sfn|Jenkins|1987|p=60}} The modern historian [[Romilly Jenkins]] states that between 695 and 717 the only competent emperors were [[Tiberius III]] ({{reign|698|705}}) and [[Anastasius II (emperor)|Anastasius II]] ({{reign|713|715}}).{{sfn|Jenkins|1987|p=60}} The crisis was ended by Emperor [[Leo III the Isaurian|Leo III]] ({{reign|717|741}}), who overthrew Theodosius, and whose dynasty reigned for 85 years.{{sfn|Jenkins|1987|p=63}}