Of the many campaigns during the Middle Ages, few are as remarkable or seemingly impossible to win at the start as the First Crusade (1095-99), and the true crowning achievement of that crusade, which resulted in two centuries of Western European Christian states in the Middle East and the permanent firing of the European imagination, was the conquest of Jerusalem on July 15, 1099 after three weeks of siege. That victorious siege came four years after the call for a crusade first went out, and had the Crusaders not taken Jerusalem, the First Crusade would not likely have been followed by any more and the campaign might have been no more than an historical footnote of what could have been.

In the 50 years following the First Crusade, the Latin states in the East were consolidated, but there were also growing tensions and new challenges. Against this backdrop, the fall of the County of Edessa in December 1144 at the hands of Zengi, the atabeg of Aleppo and Mosul, was a catastrophic event. Founded in 1098 by the future King Baldwin of Boulogne, Edessa was the first Crusader state, as well as the northernmost, weakest, and least populated. It was constantly exposed to Muslim attacks, and news of its fall reached Europe at the beginning of 1145, and the fall was the catalyst for the Second Crusade. Furthermore, the Crusader states were also concerned about their relationship with the Byzantine Empire. Relations with the Empire were often tense. As early as 1105, Bohemond of Antioch travelled to the West in search of troops to fight the Muslims while concealing his intention to attack parts of the Byzantine Empire. He reportedly obtained papal acquiescence, though not an explicit bull.

Historically, writers have given little prominence to the Second Crusade because of its failure, and it is understandably overshadowed by the triumphant First Crusade and the more captivating Third Crusade.