[3] Around 1035/7, William journeyed to Southern Italy with his brothers Drogo and Humphrey, to strengthen the ranks of Rainulf Drengot, Count of Aversa, the first Norman mercenary lord who had been granted a fief.

During the siege of Syracuse, led by the Byzantine general Georgios Maniakes, William charged and single-handedly killed the emir of the city, gaining himself his nickname, "Iron Arm".

[1][3][4] After the siege of Syracuse, due to unpaid wages and an unfair division of loot, the Norman forces were dissatisfied with Maniakes.

Maniakes, however, humiliated Arduin,[a] and both the Normans and Lombards immediately left the Byzantine expedition (which was already weakened by internal discord) and returned to Guaimar IV.

Arduin was refused as a figurehead by the Apulian Lombards, and his successors, in order Atenulf of Benevento and Argyrus, were both subjects of bribery by the Byzantines.

His title and territories were recognized by Guaimar IV, who gave William the hand of his niece Guida (the daughter of Guy of Sorrento) in marriage, and also granted him Ascoli as a personal fief.