Eustace III was the count of Boulogne from 1087 succeeding his father, Eustace II. He joined the First Crusade, being present at Nicaea, Dorylaeum, Antioch, and Jerusalem. After fighting in the battle of Ascalon, he returned home. Initially offered the Kingdom of Jerusalem, Eustace was at Apulia when he received news of Baldwin of Bourcq's election to the throne. On his return to Boulogne, he founded a Cluniac monastery in Rumilly, retired as a monk, and died in 1125.

The siege of Nicaea was the first major battle of the First Crusade, taking place from 14 May to 19 June 1097. The city was under the control of the Seljuk Turks who opted to surrender to the Byzantines in fear of the crusaders breaking into the city. The siege was followed by the Battle of Dorylaeum and the Siege of Antioch, all taking place in modern Turkey.