The Second Crusade (1147-1149) was instigated in response to the siege of Edessa, in 1144, by Zengi, the atabeg of Mosuland Alepo. European leaders were appealed by St. Bernard de Clairvaux to take up the cross, reclaiming the Christian domination in the Holy Land, the Baltic and the Iberian Peninsula. Pope Eugene III, the King of France Louis VII, and the King of Germany Conrad III, responded positively and the Second Crusade was launched.
In 1147, the two armies of Louis VII and Conrad III arrived separately in Constantinople to take the road to Jerusalem. In 1148, the crusaders arrived at Jerusalem, where the Council of Acre took place. The Haute Cour of Jerusalem met with the crusaders to decide on the best target in order to take over the Muslim forces. After a spectacular meeting, the Council decided that it was to the crusaders' best interest to attack the city of Damascus. Damascus used to be an ally of the Kingdom of Jerusalem, but over time it had shifted its commitment to Zengi. After assembling nearly 50,000 soldiers, the crusaders marched on to Damascus.
In spite of the initial enthusiasm and the great vision, the losses to the Christian armies in Damascus were overwhelming. Apparently, the Muslim forces were prepared for the attack. Some Syriac Christian historians claim that the Byzantine Emperor Manuel I Comnenus furtively delayed the crusaders' advancement to Damascus by ordering the Turks to attack them in Anatolia. When the crusaders arrived at Damascus, they were already divided and disheartened. Upon the arrival of Nur al-Din, the crusaders were easily defeated by Muslim forces.
After the defeat in Damascus, a new plan was crafted to capture Ascalon. However, Conrad received no help because the other crusaders were demoralized from the large defeat. Conrad took the road back to Constantinople, while Louis remained to Jerusalem until 1149.
In Europe, St. Bernard de Clairvaux was disgraced by the defeat. Considering that he owed an apology to the Pope, he included it to his 'Book of Consideration', where he attributes the causes of failure to the sins of the crusaders.
The aftermath of the Second Crusade had multiple layers. Except from being a great victory for the Muslims that ultimately led to the siege of Jerusalem by Saladin in 1187, it turned the momentum against the Latin East. Many historians consider that the ill-decided attack of Damascus, in effect, led to the collapse of the Christian Western Europe after the defeat at the Battle of Hattin in 1187. Damascus was taken over by Nur al-Din in 1154. King Baldwin III captured Ascalon in 1153 bringing Egypt into the picture and occupying Cairo in 1160. The relations between Jerusalem and the Byzantine Empire were ambiguous and the role of Europe was unclear, particularly after the defeat in the Second Crusade. In 1171, Saladin was pronounced Sultan of Egypt and Syria and, for the first time, the Kingdom of Jerusalem was surrounded by Muslim territories. Finally, in 1187, Saladin besieged Jerusalem, practically forcing Pope Gregory VIII to preach the Third Crusade.