The County of Edessa was one of the Crusader states

Crusader states

The Crusader states were a number of mostly 12th- and 13th-century feudal states created by Western European crusaders in Asia Minor, Greece and the Holy Land , and during the Northern Crusades in the eastern Baltic area...

 in the 12th century, based around Edessa

Edessa, Mesopotamia

Edessa is the Greek name of an Aramaic town in northern Mesopotamia, as refounded by Seleucus I Nicator. For the modern history of the city, see Şanlıurfa.-Names:...

, a city with an ancient history and an early tradition of Christianity.

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

Principality of Antioch

The Principality of Antioch, including parts of modern-day Turkey and Syria, was one of the crusader states created during the First Crusade.-Foundation:...

. Half of the county, including its capital, was located east of the Euphrates

Euphrates

The Euphrates is the longest and one of the most historically important rivers of Western Asia. Together with the Tigris, it is one of the two defining rivers of Mesopotamia...

, far to the east of the others, rendering it particularly vulnerable.