The County of Edessa was one of the Crusader states in the 12th century, based around Edessa, a city with an ancient history and an early tradition of Christianity.[citation needed]
In Late Byzantine Empire times Edessa became the center of intellectual life of the Syriac Orthodox Church. As such it also became the center for the translation of Ancient Greek philosophy into Syriac language, which provided a stepping stone for the subsequent translations into Arabic language. When the Crusades arrived, it was still an important city, enough to tempt a side-expedition after the Siege of Antioch.
The firsts Count of Edessa became King of Jerusalem, and subsequent Counts were their cousins. Unlike the other Crusader states, the County was landlocked; it was remote from the other states and was not on particularly good terms with its closest neighbor, the Principality of Antioch. Half of the county, including its capital, was located east of the Euphrates, far to the east of the others, rendering it particularly vulnerable. The part west of the Euphrates was controlled from the stronghold of Turbessel. The eastern border of Edessa was the Tigris River, but the County may not have extended quite that far.

In 1100, Baldwin became King of Jerusalem when his brother Godfrey of Bouillon died. The County of Edessa passed to his cousin Baldwin of Bourcq. He was joined by Joscelin of Courtenay, who became lord of the fortress of Turbessel on the Euphrates, an important outpost against the Seljuk Turks.
Baldwin II quickly became involved in the affairs of northern Syria and Asia Minor. He helped secure the ransom of Bohemond I of Antioch from the Danishmends in 1103, and, with Antioch, attacked the Byzantine Empire in Cilicia in 1104. Later in 1104, Edessa was attacked by Mosul, and both Baldwin and Joscelin were taken prisoner when they were defeated at the Battle of Harran. Bohemond's cousin Tancred became regent in Edessa (although Richard of Salerno actually governed the territory), until Baldwin and Joscelin were ransomed in 1108. Baldwin had to fight to regain control of the city; Tancred was eventually defeated, though Baldwin had to ally with some of the local Muslim rulers.
In 1110, all lands east of the Euphrates were lost to Mawdud of Mosul. This was not followed by an assault on Edessa itself as the Muslim rulers were more concerned with consolidating their own power.
The County of Edessa was the first of the crusader states to be established during and after the First Crusade. It was also 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. Baldwin and Joscelin were both 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. John II was succeeded by his son Manuel I Comnenus, who had to deal with consolidating power at home against his elder brothers, while Fulk was succeeded by his wife Melisende and his son Baldwin III. Joscelin had also quarreled with Raymond II of Tripoli and Raymond of Antioch, leaving Edessa with no powerful allies.

In 1144, Joscelin was able to make an alliance with Kara Aslan, the Ortoqid ruler of Diyarbakır, against the growing power and influence of Zengi. Joscelin marched out of Edessa with almost his entire army to support Kara Aslan against Aleppo. 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.