Leo III Breaks the Second Arab Siege of Constantinople
A shepherd turned general saves the capital from a year-long siege and founds a new dynasty
Quick facts
- Emperor
- Leo III
- Siege
- 717 to 718 CE
- Byzantine ally
- Bulgar Khan Tervel
- Dynasty founded
- Isaurian
What happened
Leo III, born in Syria to a family relocated to Thrace, rose through the army to become governor of the Anatolikon military province before seizing the throne in 717 CE. He inherited a Constantinople already under threat and, almost immediately, faced a massive Arab siege by land and sea. His predecessors had prepared the ground: Anastasios II had strengthened the city's walls, and Theodosios III had secured an alliance with the Bulgar Khan Tervel to guard against attacks on the land walls. Combining these preparations with his own skill in warfare and diplomacy, Leo broke the siege of 717 to 718 CE so thoroughly that it proved a disaster for the Arab caliphate rather than the conquest it had planned.
Why it matters
The failed siege ended the Umayyad Caliphate's attempt to conquer Constantinople and helped preserve Byzantium as a barrier against Arab expansion into southeastern Europe for centuries. Leo went on to found the Isaurian dynasty and reform Byzantine law, but he is remembered above all for launching Iconoclasm shortly afterward.
How we know
Leo III's rise and the 717-718 CE siege are detailed in the World History Encyclopedia's biography of him, and Dumbarton Oaks's exhibition on Byzantine imperial seals corroborates the preparations by his predecessors and the siege's failure.
Sources
- World History Encyclopedia. Leo III · Reputable sourceworldhistory.org · The domain "worldhistory.org" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- Dumbarton Oaks. Leo III (717-741) · Primary source (author-declared)doaks.org · Cited as a "primary" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
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