The Expedition to Samosata was undertaken by the future Baldwin I of Jerusalem following his ascension to co-regent of Edessa as a part of the First Crusade.
Baldwin decided upon his ascension of co-regent of Edessa that Samosata would have to be eliminated in order for his new county to fully control the surrounding countryside and to establish unbroken communications with Byzantium in the west and the crusaders sieging Antioch in the south.
[1] The Armenian, Christian inhabitants of Edessa enthusiastically supported his plan, and the main part of their military accompanied him.
The Edessenes were poor, inexperienced soldiers, and the company was quickly caught in an ambush by the Turks of Samosata, who slew around a thousand of them in the ensuing battle.
From this point, he was able to control the flow of the Turks in and out of the city, resulting in a decline in Turkish raids into the Edessene countryside.