The County of Edessa was the first of the crusader states to be established during and after the First Crusade. It dates from 1098 when Baldwin of Boulogne left the main army of the First Crusade and founded his own principality. Edessa was the most northerly, the weakest, and the least populated; as such, it was subject to frequent attacks from the surrounding Muslim states ruled by the Ortoqids, Danishmends, and Seljuk Turks. Count Baldwin II and future count Joscelin of Courtenay were taken captive after their defeat at the Battle of Harran in 1104. Joscelin was captured a second time in 1122, and although Edessa recovered somewhat after the Battle of Azaz in 1125, Joscelin was killed in battle in 1131. His successor Joscelin II was forced into an alliance with the Byzantine Empire, but in 1143 both the Byzantine emperor John II Comnenus and the King of Jerusalem Fulk of Anjou died. Joscelin had also quarreled with Raymond II of Tripoli and Raymond of Poitiers, leaving Edessa with no powerful allies.

Zengi, already seeking to take advantage of Fulk's death in 1143, hurried north to besiege Edessa, arriving on November 28. The city had been warned of his arrival and was prepared for a siege, but there was little they could do while Joscelin and the army were elsewhere. Zengi surrounded the entire city, realizing that there was no army defending it. He built siege engines and began to mine the walls, while his forces were joined by Kurdish and Turcoman reinforcements. The inhabitants of Edessa resisted as much as they could, but had no experience in siege warfare; the city's numerous towers remained unmanned. They also had no knowledge of counter-mining, and part of the wall near the Gate of the Hours collapsed on December 24. Zengi's troops rushed into the city, killing all those who were unable to flee to the Citadel of Maniaces.

News of the fall of Edessa reached Europe, and Raymond of Poitiers had already sent a delegation including Hugh, Bishop of Jabala, to seek aid from Pope Eugene III. On December 1, 1145, Eugene issued the papal bull Quantum praedecessores calling for the Second Crusade.