of whom he defeated in pitched battle. The army was mostly spared further infighting until around 273 AD, when Aurelian defeated the Gallic usurper Tetricus in the Battle of Chalons. The next decade saw an incredible number of usurpers, sometimes three at the same time, all vying for the imperial throne. Most of the battles are not recorded, due primarily to the turmoil of the time, until Diocletian, a usurper himself, defeated Carinus at the Battle of the Margus and became emperor. Some small measure of stability again returned at this point, with the empire split into a Tetrarchy of two greater and two lesser emperors, a system that staved off civil wars for a short time until 312 AD. In that year, relations between the tetrarchy collapsed for good and Constantine I, Licinius, Maxentius and Maximinus jostled for control of the empire. In the Battle of Turin Constantine defeated Maxentius, and in the Battle of Tzirallum, Licinius defeated Maximinus. From 314 AD onwards, Constantine defeated Licinius in the Battle of Cibalae, then the Battle of Mardia, and then again at the Battle of Adrianople, the Battle of the Hellespont and the Battle of Chrysopolis. Constantine then turned upon Maxentius, beating him in the Battle of Verona and the Battle of Milvian Bridge in the same year. Constantine's son Constantius II inherited his father's rule and later defeated the usurper Magnentius in first the Battle of Mursa Major and then the Battle of Mons Seleucus. Successive emperors Valens and Theodosius I also defeated usurpers in, respectively, the Battle of Thyatira, and the battles of the Save and the Frigidus. Struggle with the Sassanid Empire (230–363 AD) After overthrowing the Parthian confederacy,Grant, The History of Rome, p. 283 the Sassanid Empire that arose from its remains pursued a more aggressive expansionist policy than their predecessorsLuttwak, The Grand Strategy of the Roman Empire, p. 151 and continued to make war against Rome. In 230 AD, the first Sassanid emperor attacked Roman territory first in Armenia and then in Mesopotamia but Roman losses were largely restored by Severus within a few years. In 243 AD, Emperor Gordian III's army retook the Roman cities of Hatra, Nisibis and Carrhae from the Sassanids after defeating the Sassanids at the Battle of Resaena but what happened next is unclear: Persian sources claim that Gordian was defeated and killed in the Battle of Misikhe but Roman sources