Constantius was a member of the "Protectores Augusti Nostri" under the emperor Aurelian and fought in the east against the secessionist Palmyrene Empire. While the claim that he had been made a "dux" under the emperor Probus is probably a fabrication, he certainly attained the rank of "tribunus" within the army, and during the reign of Carus he was raised to the position of "praeses", or governor, of the province of Dalmatia. It has been conjectured that he switched allegiances to support the claims of the future emperor Diocletian just before Diocletian defeated Carinus, the son of Carus, at the Battle of the Margus in July 285.