Carus in the context of "Numerian"

Diocletian (/ˌdaɪ.əˈkliːʃən/ DY-ə-KLEE-shən; Latin: Gaius Aurelius Valerius Diocletianus; Ancient Greek: Διοκλητιανός, romanized: Diokletianós; 242/245 – 311/312), nicknamed Jovius, was Roman emperor from 284 until his abdication in 305. He was born Diocles to a family of low status in the Roman province of Dalmatia. As with other Illyrian soldiers of the period, Diocles rose through the ranks of the military early in his career, serving under Aurelian and Probus, and eventually becoming a cavalry commander for the army of Emperor Carus. After the deaths of Carus and his son Numerian on a campaign in Persia, Diocles was proclaimed emperor by the troops, taking the name "Diocletianus". The title was also claimed by Carus's surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.

Diocletian's reign stabilized the empire and ended the Crisis of the Third Century. He initiated the process of the Roman Empire split and appointed fellow officer Maximian as Augustus, co-emperor, in 286. Diocletian reigned in the Eastern Empire, and Maximian reigned in the Western Empire. Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as junior colleagues (each with the title Caesar), under himself and Maximian respectively. Under the Tetrarchy, or "rule of four", each tetrarch would rule over a quarter-division of the empire. Diocletian secured the empire's borders and purged it of all threats to his power. He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298. Galerius, aided by Diocletian, campaigned successfully against Persia, the empire's traditional enemy, and in 299, he sacked their capital, Ctesiphon. Diocletian led the subsequent negotiations and achieved a lasting and favorable peace.

Marcus Aurelius Carinus (died 285) was Roman Emperor from 283 to 285. The eldest son of the Emperor Carus, he was first appointed Caesar in late 282, then given the title of Augustus in early 283, and made co-emperor of the western part of the Empire by his father. Official accounts of his character and career, which portray him as dissolute and incompetent, have been filtered through the propaganda of his successful opponent Diocletian.

Bahram II (also spelled Wahram II or Warahran II; Middle Persian: 𐭥𐭫𐭧𐭫𐭠𐭭) was the fifth Sasanian King of Kings (shahanshah) of Iran, from 274 to 293. He was the son and successor of Bahram I (r. 271–274). Bahram II, while still in his teens, ascended the throne with the aid of the powerful Zoroastrian priest Kartir, just like his father had done.

He was met with considerable challenges during his reign, facing a rebellion in the east led by his brother, the Kushano-Sasanian dynast Hormizd I Kushanshah, who also assumed the title of King of Kings and possibly laid claims to the Sasanian throne. Another rebellion, led by Bahram II's cousin Hormizd of Sakastan in Sakastan, also occurred around this period. In Khuzestan, a Zoroastrian factional revolt led by a high-priest (mowbed) occurred. The Roman emperor Carus exploited the turbulent situation of Iran by launching a campaign into its holdings in Mesopotamia in 283. Bahram II, who was in the east, was unable to mount an effective coordinated defense at the time, possibly losing his capital of Ctesiphon to the Roman emperor. However, Carus died soon afterwards, reportedly being struck by lightning. As a result, the Roman army withdrew, and Mesopotamia was reclaimed by the Sasanians. By the end of his reign, Bahram II had made peace with the Roman emperor Diocletian and put an end to the disturbances in Khuzestan and the east.