Second Siege of Constantinople

- When: Summer of 717 - August 718
- Adversaries: Arab Caliphate of the Umayyads vs. Byzantine and Bulgarian empires
- Arabian land forces: 120,000 soldiersChristian Classics: From Decline and Fall of the Roman Empire
- Arabian sea forces: 1800 ships, each manned with up to 100 soldiersChristian Classics: From Decline and Fall of the Roman Empire
- Number of Arabian soldiers that managed to retreat: Approx. 30,000The Latin Library at Ad Fontes Academy: Islam and Europe Timeline
- Number of Arabian ships that managed to retreat: 5Christian Classics: From Decline and Fall of the Roman Empire
- Outcome: Arabs retreat, August 718

The second siege of Constantinople began in the summer of 717. Almost forty years after the five year long first siege, the Arab Caliphate of the Umayyads besieged Constantinople the second time by both land and sea. The Arab forces retreated in August 718, after being unable to breach the city's defenses.
Arab Backstory

The Arab Empire had continued to expand during the 7th century. Caliph Suleiman was determined to do what his predecessor had failed to do in 674 — capture Constantinople.
Byzantine Backstory

The Byzantines found themselves menaced by Avars, Bulgarians and Arabs when the new emperor, Leo III, ascended to the throne in March 717. Leo used diplomacy to make allies of the Bulgarians.Christian Classics: From Decline and Fall of the Roman Empire
Preparation for War

The Arab Caliphate sent an army by land, through Anatolia, to attack Constantinople from the north. They also sent a fleet of 1,800 ships to attack from the south.
Siege

The Bulgarians fought the Arab forces on land.Christian Classics: From Decline and Fall of the Roman Empire On the sea, the Arab fleet, constantly under fire by the Byzantines, was unable to reach Constantinople. The extremely harsh winter of 717-718 devastated the Arab army. The reinforcement of 760 ships that arrived in the spring of 718 was not sufficient to turn the tables, and the Arabs retreated in August 718.Christian Classics: From Decline and Fall of the Roman Empire
Technology

Just like during the first siege, Greek fire played a decisive role in the defense of Constantinople.University of Calgary: Old World Contacts - Greek Fire An incendiary device of uncertain chemical composition, Greek fire was especially effective in repulsing the enemy ships. During the second siege, historians also mention a chain that barred the Arab ships from coming too close to the city.Christian Classics: From Decline and Fall of the Roman Empire
Significance

The determination of the Byzantine Empire to stand firm in the face of Arab expansion is considered to be of primary importance for the shaping of Europe, giving the continent 700 years to prepare for the new wave of Muslim incursions.The Latin Library at Ad Fontes Academy: Islam and Europe Timeline Constantinople would ultimately fall to the Turks in 1453.