Siege of Baghdad ( 812 – 813 ) The siege of Baghdad was a part of a civil war between al - Amin and al - Ma ' mun for the Abbasid Caliphate of Baghdad . The siege lasted from August 812 until September 813 . The siege is described in great detail by Muhammad ibn Jarir al - Tabari in his famous " Tarikh al - Tabari " ( History of the Prophets and Kings ) . Initial battles en route to Baghdad . After the defeat of Caliph al - Amins ' army at the Battle of Rayy and the death of his commander Ali ibn Isa ibn Mahan , the armies of al - Amin were in retreat moving west from Iran to Iraq back to their base camp at Baghdad . Future Caliph Al - Ma ' mun ' s general Tahir ibn Husayn , the victor of Battle of Rayy decided to chase the retreating army . However , reinforcements from Baghdad arrived under the able leadership of Abd al - Rahman ibn Jabala . Abd al - Rahman decided to fortify himself behind the walls and gates of Hamadan . But when Tahir ibn Husayn came closer to the city , Abd al - Rahman decided to come out and meet this threat head on . Twice Abd al - Rahman was driven back into the city . Tahir ibn Husayn began a blockade of the city and Abd al - Rahman ’ s forces began to shower arrows and throw stones from the city walls . Eventually Abd al - Rahman was obliged to leave and ask for terms due to resentment of the people of Hamadan at the occupation and rapidly depleting supplies . Tahir ibn Husayn , realizing that Abd al - Rahman ibn Jabala had left the city , decided not to waste further time in Hamadan and marched westward towards Baghdad . On his way , he reached a certain pass named Asadabad where his army was ambushed by Abd al - Rahman ibn Jabala . The surprise attack caught Tahir ’ s troops off guard . But because the army was well disciplined the infantry managed to hold them off until Tahir ibn Husayn ’ s cavalry was ready to attack . In the ensuing confusion Abd al - Rahman ibn Jabala who had dismounted his horse was killed and his army defeated . After the short delay , Tahir ibn Husayn began his march towards Baghdad again . The news of the defeat of first Ali ibn Isa ibn Mahan and now Abd al - Rahman ibn Jabala reached Caliph al - Amin and greatly distressed him . It seemed to the people of Baghdad that Tahir ibn Husayn was unstoppable . Nevertheless , people in al - Ahwaz under the leadership of Muhammad ibn Yazid al - Muhallabi put up a fierce resistance to the armies of Tahir ibn Husayn . After defeating the army of al Muhallabi , Tahir reached the gates of Baghdad and at the right time too as his reinforcements arrived under the leadership of Harthama ibn A ' yan . The Siege . The Siege has no parallel in warfare of the time . Although the city was surrounded by walls most of the population lived in suburbs which were not . The siege was therefore not an attack on a fortified perimeter but rather street fighting , house to house invasion as well as temporary improvised fortifications . It was extremely destructive especially for the civilian population . Immediately , Tahir ibn Husayn ordered the other commanders namely Zuhayr ibn al - Musayyab al - Dabbi and Harthama ibn A ’ yan to set up camps at " Qasr Raqqat Kalwadha " and " Nahr Bin " respectively , while he set up camp at " Al - Anbar Gate " . They set up siege engines , mangonel s , and dug trenches . Both sides are known to have used siege weapons . At one point , a general of Al - Amin known as al - Samarqandi used boats to transport mangonels on the river Tigris and bombard enemy positions in the suburbs of Baghdad inflicting more damage to the civilian population than to the besiegers . There were several vicious battles , such as at al - Amin ' s palace of " Qasr Halih " , at " Darb al - Hijarah " and " al - Shammasiyyah Gate " . In the later stages of the siege irregulars came to the aid of Al - Amin . As things were getting worse and Tahir ibn Husayn pushed into the city , Al - Amin sought to negotiate safe passage out . Tahir ibn Husayn reluctantly agreed on the condition that Al - Amin turn over his scepter , seal and other signs of being Caliph . Al - Amin reluctant to do so tried to leave on a boat . Tahir ibn Husayn noticed the boat and sent his men after Al - Amin who was captured and brought to a room where he was executed . His head was placed on the " Al - Anbar Gate " . Muhammad ibn Jarir al - Tabari quotes Tahir ' s letter to the new Caliph Al - Ma ' mun informing him of Al - Amin ' s capture and execution and the state of peace resulting in Baghdad . Aftermath . The end result was that Al - Ma ' mun became the new Abbasid Caliph . Yet , he would not arrive in Baghdad until 819 due to the destruction and continued disturbances in the city .