The prosperity of the Kingdom of Jerusalem led to a weakening of the military spirit, and internal strife crippled the resources of the kingdom. On December 24, 1144, the capture of the strong frontier fortress of Edessa by Zengi of Mosul inflicted a serious blow on the Christian power.
The news of the fall of Edessa led Pope Eugenius III to issue the bull Quantum praedecessores on December 1, 1145, calling for a second crusade. This was at first ignored, although Louis VII of France had also been considering a new crusade independently of the Pope. The crusade was preached in France by Bernard of Clairvaux, the "honey-tongued teacher" who found it expedient to dwell upon the taking of the cross as a potent means of gaining absolution for sin and attaining grace. Bernard also preached to Conrad III of Germany in December of 1146, delivering an emotional sermon in which he took the role of Christ and asked what more he could do for the emperor. Meanwhile, other German princes extended the idea of a Crusade to the Slavic tribes living to the northeast of the Holy Roman Empire, and were authorized to launch the Wendish Crusade against them.
The Pope also authorized a Crusade in Spain, although the war against the Moors had been going on for some time already. In 1147 an English fleet on its way to the Mediterranean captured Lisbon from the Moors, and went on to capture Tortosa in 1148. However, the fleet never arrived in Syria.
The French and German Crusaders left by land in May, 1147. Conrad's army arrived in Constantinople first, but relations with Byzantine emperor Manuel I Comnenus were poor and the Germans were convinced to cross into Asia Minor as quickly as possible. There, they decided not to wait for the French, and marched towards Edessa. Conrad split his army into two divisions; one of these was destroyed by the Seljuk Turks in October, 1147. The other division was similarly massacred early in 1148.
Manuel's relations with the French army were somewhat better, but he refused to help them with his own troops and had them swear to return to the Empire any territory they captured. They met the remnants of Conrad's army at Nicaea, but the French suffered a heavy defeat in 1148 just as the Germans had. The remnants of both armies eventually made their way to Syria by sea.
In Jerusalem Conrad convinced Baldwin III to attack Damascus, despite the fact that the Crusader kingdom had a truce with the city. The other Crusaders wanted to attack the weaker Aleppo, the capture of which would have provided easier access to Edessa and probably limited Nur ad-Din's growing power. The siege of Damascus began in July, 1148, though the Crusaders were encamped on a waterless plain and ended up withdrawing after less than a week. Conrad and Louis returned home in failure.
As a result of the Crusade, Damascus no longer trusted the Crusaders, and the city was handed over to Nur ad-Din in 1154. Baldwin III unwisely seized Ascalon in 1153 and brought Egypt into the sphere of conflict, thus preparing the way for the fall of Jerusalem. Bernard of Clairvaux was also humiliated, and when his attempt to call a new Crusade failed, he tried to disassociate himself from the fiasco of the Second Crusade altogether. He died in 1153.