Persia
His main concern as the Emperor's principal minister and adviser was in dealing with the threat to the oriental provinces posed by the renascent power of Persia under one of its most effective "Kings of Kings", Shapur I.
Shapur's ambitions when he succeeded his father Ardashir in 240 were no doubt inflated by his initial successes, but there also seems no doubt that he was determined to: (i) secure strategic control of the minor states of eastern Mesopotamia that controlled access to Roman Syria across the eastern desert frontier west of the River Euphrates; and (ii) replace Rome as the hegemonic power in the Kingdom of Armenia. During the reign of Maximinus, Rome had suffered the loss of considerable territories in Mesopotamia to Ardashir which the Roman Emperor had been unable to prevent or avenge because of his internal distractions: on his accession, Shapur renewed the onslaught, capturing more of the Mesopotamian fortresses and penetrating Syria itself, where Antioch, the capital of the Roman east, may have come under threat. More seriously, perhaps, the confidence of Rome's governing elite that the Empire was capable of seeing off the Persian threat to the Oriens was seriously undermined.
In the first two years of his prefecture, Timesitheus was not able to give his attention to the threat to Rome's territories in the east posed by Shapur, but in 242 he began to organise a response appropriate to the magnitude of the crisis. Under his supervision, a powerful army was put together consisting of vexillationes from the garrisons of the Rhine and Danube provinces. This expeditionary force seems to have been very well-equipped and financed. Neither did Timesitheus neglect the issue of morale. For the last time in recorded history, war was declared with traditional Roman formalities from the temple of Janus. More significantly, before he left Rome with the Emperor, Timesitheus addressed the concerns of the Greek east by holding games in honour of Pallas Athena in her capacity as Athena Promachos - Aθηνᾶ Πρόμαχος (Athena Who-Fights-In-The-Frontline) - the patron goddess of Athens credited with saving Greece from Persia at the time of the Battle of Marathon. The object of this latter exercise was probably to reaffirm the role of Rome under the Emperor Gordian as the heir of Athens in securing the Greek world from Persian domination.
The removal of so many seasoned troops from their Rhine and Danube stations encouraged an assault across the lower Danube by the Carpi and other northern barbarians. However, Timesitheus, en route to the east through the Balkans, inflicted a serious defeat on the invaders in Thracia. He seems then to have followed the usual practice of Roman commanders after victories over barbarian peoples of obliging the defeated to provide contingents of troops. Such measures were intended not only to reinforce his army, but also to remove those restless young men who might have been disposed to make more trouble in its absence.
On arriving in the theatre of operations he seems to have mounted a highly successful campaign against the Persians in Mesopotamia, inflicting a crushing defeat on them at the Battle of Resaena (Ras-al Ayn, Syria). This enabled the Romans to recover all their main positions in Mesopotamia, including Carrhae, Nisbis and Singara and restore their colony at Edessa in Adiabene. The SHA suggests that it was Timesitheus' intention to follow up this success by advancing on the Persian western capital at Ctesiphon. His death meant that Shapur never had to face a powerful, well-equipped Roman army, led by a first-class general and not distracted by other enemies (as in the case of Valerian in 260) until he encountered Odenathus of Palmyra.
