History of the Crusades/The Second Crusade

Background
In 1144, Edessa was captured by Zengi, the Atabeg of Mosul, bringing the city back under Muslim control. The shock of the fall of one of the Crusader states sparked the call for the Second Crusade. The leaders of the Second Crusade were king Louis VII of France and Conrad III of Germany. Though Zengi had died by the time the crusade was launched, his successor, Nur ad-Din continued to be a threat.

The Events of the Crusade
The Germans traveled via the overland route, stopping in Constantinople. Relations with the Eastern Roman Empire were rocky and Conrad decided to leave Constantinople as soon as possible instead of waiting for the French. Conrad's march through Anatolia was poorly planned and ended in disaster. Nearly his entire army was wiped out and Conrad was forced to flee back to Constantinople. When the French arrived, they regrouped with the Germans and continued to attempt to overland route, but they did not receive assistance from Constantinople and the French decided that they would go by sea instead.

Upon arriving in Jerusalem, the plans for the crusade changed. Rather than going north to retake Edessa, it was decided that the crusaders would instead attack Damascus. Damascus was under Muslim rule but they not under Nur ad-Din's control. However, it was decided that the best way to keep Damascus out of Nur ad-Din's hands was to capture Damascus for the Christians rather than continue to pursue an alliance with Damascus against Nur ad-Din.

The assault on Damascus was a complete failure and only served to alienate Damascus from the Christians and drive them into the hands of Nur ad-Din, who had arrived to relieve the city. The long term effects of the crusade were negative for the Christians. The crusade had accomplished little but foster disagreement and resentment between the Christians and had helped to unify the Muslims instead.