In this capacity, Harith fought on behalf of the Byzantines in all their numerous wars against Persia. Already in 528 he was one of the commanders sent in a punitive expedition against Mundhir. In 529, he helped suppress the wide-scale Samaritan revolt, capturing 20,000 boys and girls whom he sold as slaves. It was perhaps Harith's successful participation in this conflict that led Justinian to promote him to supreme phylarch. It is possible that he took part with his men in the Byzantine victory in the Battle of Dara in 530, although no source explicitly mentions him. In 531, he led a 5000-strong Arab contingent in the Battle of Callinicum. Procopius, a source hostile to the Ghassanid ruler, states that the Arabs, stationed on the Byzantine right, betrayed the Byzantines and fled, costing them the battle. John Malalas, however, whose record is generally more reliable, reports that while some Arabs indeed fled, Harith stood firm. The charge of treason leveled by Procopius against Harith seems to be further undermined by the fact that, unlike Belisarius, he was retained in command and was active in operations around Martyropolis later in the year.