The siege of Aleppo in April 1138 was a significant attempt to capture the city by the allied forces of the Byzantines and the Franks.^[1]

The Byzantine Emperor John II Komnenos allied with the Franks in an attempt to capture Aleppo.^[1] The Christian army was largely composed of Byzantine regulars and also included a Templar force and substantial contingents from Antioch and Edessa.^[1] As the Christian army approached Aleppo its inhabitants withdrew into the outlying garrisons and sent word to Zengi, asking him for help.^[1] Zengi rushed to obtain reinforcements before the arrival of the allied army, he received a reinforcement of cavalry, infantry and specialist archers just in time.^[1]

The Byzantines were aware of the strategic importance of Aleppo and one of the objectives of their Syrian campaign was to create a Christian buffer state centred on Aleppo but also including Shaizar, Homs and Hama.^[1] Due to the dangers involved the Byzantines were content to let the Franks own the buffer state of the hinterlands, presumably under imperial suzerainty.^[1]

The Byzantines were camped on Queiq river and launched attacks on the south and west of Aleppo on April 19 in an attempt to size out the strength of the garrison and intimidate them with the size and aggression of the besieging force.^[1] Instead the reverse happened, large numbers of the Muslim militia made a sortie against the Byzantines and emerged victorious from the skirmishing.^[1] One of the senior Byzantine commanders was wounded during the fight.^[1] Following their repulse, the Christian army departed in search of easier pickings. The siege is hardly mentioned in Christian chronicles, and while Aleppo might have been viewed as a target, if its defences had been weak, there is evidence that the city of Shaizar was the real goal of the allied army. After taking a number of towns by assault John II's army unsuccessfully besieged Shaizar.^[5][2]