In 1130 he allied with Taj al-Mulk Buri of Damascus against the crusaders, but this was only a ruse to extend his power; he had Buri's son taken prisoner and seized Hama from him. He also besieged Hims, the governor of which was accompanying him at the time, but could not capture it, so he returned to Mosul, where Buri's son and the other prisoners from Damascus were ransomed for 50 000 dinars. In 1131 Zengi agreed to return the 50 000 dinars if Buri would deliver to him Dubais, emir of al-Hilla in Iraq, who had fled to Damascus to escape al-Mustarshid. When an ambassador from the caliph arrived to bring Dubais back, Zengi attacked him and killed some of his retinue; the ambassador returned to Baghdad without Dubais.
In 1134 Zengi became involved in Ortoqid affairs, allying with the emir Timurtash (son of Ilghazi) against Timurtash's cousin Da'ud. Zengi's real desires, however, lay to the south, in Damascus. In 1135 Zengi received an appeal for help from Ismail, who had succeeded his father Buri as emir of Damascus, and who was in fear for his life from his own citizenry who considered him a cruel tyrant. Ismail was willing to surrender the city to Zengi in order to restore peace. None of Ismail's family or advisors wanted this, however, and Ismail was murdered by his own mother, Zumurrud, to prevent him from turning over the city to Zengi's control. Ismail was succeeded by his brother Shihab ad-Din Mahmud.
Zengi was not discouraged by this turn of events and arrived at Damascus anyway, still intending to seize it. The siege lasted for some time with no success on Zengi's part, so a truce was made and Shahib ad-Din's brother Bahram-Shah was given as a hostage. At the same time, news of the siege had reached the caliph and Baghdad, and a messenger was sent with orders for Zengi to leave Damascus and take control of the governance of Iraq. The messenger was ignored but Zengi gave up the siege, as per the terms of the truce with Shahib ad-Din. On the way back to Aleppo, Zengi besieged Hims, whose governor had angered him, and Shahib ad-Din responded to the city's call for help by sending Mu'in ad-Din Unur to govern it.
In May 1138 Zengi came to an agreement with Damascus. He married Zumurrud, the same woman who had murdered her son Ismail, and received Hims as her dowry. In July 1139 Zumurrud's surviving son, Shihab ad-Din, was assassinated and Zengi marched on Damascus to take possession of the city. The Damascenes, united under Mu'in ad-Din Unur, acting as regent for Shihab ad-Din's successor Jamal ad-Din, once again allied with Jerusalem to repel Zengi. Zengi also besieged Jamal ad-Din's former possession of Baalbek, and Mu'in ad-Din was in charge of its defenses as well. After Zengi abandoned his siege of Damascus, Jamal ad-Din died of a disease, and was succeeded by his son Mujir ad-Din, with Mu'in ad-Din remaining as regent.
Mu'in ad-Din signed a new peace treaty with Jerusalem for their mutual protection against Zengi. While Mu'in ad-Din and the crusaders joined together to besiege Banias, Zengi once more laid siege to Damascus, but quickly abandoned it again. There were no major engagements between the crusaders, Damascus, and Zengi for the next few years, but Zengi in the meantime campaigned in the north and captured Ashib and the Armenian fortress of Hizan.
In 1144 Zengi besieged the crusader County of Edessa (see Siege of Edessa). Edessa was the weakest and least Latinized crusader state, and Zengi captured it on December 24, 1144. This event led to the Second Crusade, and later Muslim chroniclers noted it as the start of the jihad against the Crusader states.
Zengi's sudden death threw his forces into a panic. His army disintegrated, the treasury was looted, and the crusader princes, made bold by Zengi's demise, plotted to attack Aleppo and Edessa. Mu'in ad-Din immediately recaptured Baalbek, Hims, and other territories lost to Zengi over the years.
According to Crusader legend, Zengi's mother was Ida of Austria (mother of Leopold III of Austria), who had supposedly been captured during the Crusade of 1101 and placed in a harem. She was 46 in 1101, Zengi was born in 1085, and his father died in 1094 so this is not feasible.
Zengi was courageous, strong in leadership and a very skilled warrior according to all of the Islam chroniclers of his day. The conquest of Edessa being his greatest achievement. These same chroniclers however, also relate Zengi as being a very violent, cruel, and brutal man. Muslims, Byzantines, and Franks all suffered at his hands.
Unlike Saladin at Jerusalem in 1187, Zengi did not keep his word to protect his captives at Baalbek in 1139. According to Ibn al-‘Adim, “He (Zengi) had sworn to the people of the citadel with strong oaths and on the Qur’an and divorcing (his wives). When they came down from the citadel he betrayed them, flayed its governor and hanged the rest.” (Source: Ibid. Also, Ibn Wasil, Mufarrij al-Kurub, p. 86)
“The atebeg was violent, powerful, awe-inspiring and liable to attack suddenly… When he rode, the troops use to walk behind him as if they were between two threads, out of fear they would trample over crops, and nobody out of fear dared to trample on a single stem (of them) nor march his horse on them… If anyone transgressed, he was crucified. He (Zengi) used to say: ‘It does not happen that there is more than one tyrant (meaning himself) at one time.’” By Ibn al-‘Adim (Source: Ibn al-‘Adim, Zubda, vol. 2, p. 471)
“He (Zengi) was tyrannical and he would strike with indiscriminate recklessness. He was like a leopard in character, like a lion in fury, not renouncing any severity, not knowing any kindness… He was feared for his sudden attacking; shunned for his roughness; aggressive, insolent, death to enemies and citizens.” By Imad ad-Din al-Isfahani (Source: Al-Bundari, Zubdat al-nusra, ed. M.Y. Houtsma (Leiden, 1889), p. 205)
“When he (Zengi) was unhappy with an emir, he would kill him or banish him and leave that individual’s children alive but castrate them. Whenever one of his pages pleased him by his beauty he would treat him in the same way so that the characteristics of youth would last longer in him.” By Imad ad-Din al-Isfahani (Source: The Second Crusade – Scope & Consequences Edited by Jonathan Phillips & Martin Hoch)