Imad al-Din Zengi gained recognition as a powerful warlord in 1126, in the conflict between the Abbasid Caliph and the Sultan of Baghdad. Zengi fought on the side of the Sultan and defeated the forces of the Caliph. By conducting himself tactfully, he earned the trust of both the Caliph and the Sultan. With their support he became the atabeg of Mosul in 1127. Till 1130, Zengi exploited the factionalism in Northern Syria to defeat a number of Islamic warlords and seized control of towns like Aleppo, Homs, and Shayzar.
With his exploits in the battlefield, and the cruelty and capriciousness that he showed in his personal dealings, Zengi inspired fear in his political rivals. In the 1130s, he was the most dreaded and powerful figure in Outremer. During this period he made several attempts to capture Damascus, which had an alliance with the Kingdom of Jerusalem, then under King Fulk.
In December 1139, Zengi laid a siege to Damascus. He did not dare to launch a full scale attack, probably because the city was of great historical and religious significance. He preferred to force Damascus into submission through the tactics of economic chokehold. Instead of yielding to Zengi, the ruler of Damascus Mu'in ad-Din Unur turned towards his non-Islamic ally, King Fulk, for help. Fulk dispatched his army to Damascus and Zengi was forced to lift the siege and retreat to Mosul. Throughout the 1130s, Zengi had shown little interest in attacking the crusader states, but that changed in the 1140s.
On April 1, 1143, Emperor of the Byzantine Empire John II Komnenos died when he was injured by a poisoned arrow while hunting wild boar. In the same year, King Fulk died after falling from his horse while he was hunting rabbits. These two hunting deaths led to the Byzantine Empire and Kingdom of Jerusalem getting bogged down in a succession crisis.
Zengi took advantage of the chaos to attack the Kurdish warlords in Diyar Bakr. One of these Kurdish warlords had a mutual defense treaty with Jocelyn II, count of Edessa. Edessa was the first crusader state, established by Baldwin of Boulogne on 10 March 1098. Jocelyn II had little sense of Zengi’s capabilities and intentions. He thought that Edessa was safe from attacks, and marched out with a large force (almost his entire army) to help his Kurdish ally. Zengi had informants inside Edessa. When he learned that Edessa was largely undefended in Jocelyn’s absence, he force-marched his troops to the city.
With continuous bombardment and use of armored seize towers, Zengi devastated the life of the city’s inhabitants. Jocelyn tried to muster a rescue army to save his city, but it was too late. Zengi’s miners collapsed a section of the city’s walls on 24 December 1144, and his troops rushed into the city. The Christian inhabitants fled towards the city’s citadel, but such was the rush that hundreds got crushed to death (among them was the Latin archbishop). Zengi’s soldiers massacred the men and enslaved the women and children. Edessa was lost to the crusaders—the first of the four crusader states to fall.
The fall of Edessa and the reports of the massacre and enslavement of the city’s inhabitants sent shock waves through the remaining three crusader states and Western Europe. In 1145, Pope Eugenius III launched the Second Crusade. In 1147, the armies of Louis VII of France and Conrad III of Germany left for Outremer. The could not have the satisfaction of extracting vengeance from Zengi for his destruction of Edessa because he was assassinated in September 1146 by one of his Frankish slaves. Zengi’s territories were divided between his two sons: Saif ad-Din (who got Mosul and eastern territory) and the younger son Nur ad-Din (who got Aleppo and western territory).
Nur ad-Din became a powerful ruler. With ruthless and shrewd military action, he destroyed the Second Crusade. During his reign, between 1146 to 1174, he subjugated large parts of Asia Minor and Egypt.