Late Roman Field Armies

[ValentinianVictrix]

There were at least three field armies in the East during Constantius' reign- under the commands of these magister's- Magister Militum per Orientem, Magister Militum per Thracias and the Magister Militum per Illyricum. Valen's himself commanded the Army of Thrace when he crossed the Danube in three campaigns against the Greuthungi Goths. Then there was the field army in Italy and at least one in Gaul.
Of course one of the great 'what if's' in the combining of field armies would have been if Gratian had reached Adrianople in time to combine his army with that of Valens (elements of Gratian's army were with Valens at the Battle of Adrianople as they had been despatched with Richomeres ahead of Gratian's main force).