Aurelian defeated the peoples who were attacking the empire. He defeated the Alemanni (after a devastating war) Goths, Vandals, Juthungi, Sarmatians, and Carpi. He also re-unified the empire by defeating two breakaway parts: the Palmyrene Empire in the Middle East and the Gallic Empire in Gaul and Britannia.
Neither Aurelian, nor next few emperors, restored full stability. He himself was murdered because he was tough on corruption. Two of the next three emperors were also murdered. The next emperor and his sons managed to gain further stability for the empire. Then Diocletian’s decided that one man could not rule the vast empire alone because of the murders of previous emperors and because there was tension in nearly all the provinces of the empire. Therefore, he created the tetrarchy (rule by four). He subdivided the empire into four administrative units and put two senior emperors and two junior emperors in charge of them. This system provided stability during his reign. However, when he retired these men fought each other. Constantine I fought two civil wars, became the sole emperor and restored stability.