Diocletian and Maximian both retired as Augusti in 305, in favor of Constantius and Galerius. However, when Constantius died the following year, his son Constantine was proclaimed emperor by the Western legions without seeking Galerius’ approval. Although Galerius eventually acknowledged Constantine, the move encouraged Maximian’s son Maxentius to seize power in Italy.
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In the spring of 299 Diocletian met Galerius at Nisibis, where they laid out peace terms to present to Shah Narseh of Persia. Their demands included the cession of lands east of the Tigris to Rome, the restoration of Armenia as a Roman client, and the establishment of the Roman city of Nisibis as the sole point of trade between the two empires. Narseh accepted all these terms and in return had his wife and the other hostages seized at the Battle of Satala returned to him. It would be another four decades before Persia would challenge the Roman Empire again.
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In c. 300 the Himyarite king Shammar Yahar’ish (r. c. 286–311), son of Yasirum Yuhan’im, invaded the Kingdom of Hadramawt and seized its capital Shabwa. The conquest of Hadramawt united all Yemen under Himyarite rule, prompting the Himyarite rulers to extend their titles to “kings of Saba’, dhu-Raydan (Himyar), Hadramawt, and Yamnat (the South)”.
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In February 303 Diocletian began a campaign of persecution against the growing Christian population of the Roman Empire, ordering the destruction of churches and scriptures, the arrest and imprisonment of church leaders, and the deprivation of Christians of their legal rights and ranks. Hundreds, possibly thousands, were also executed for refusing to comply with demands to sacrifice to the Roman gods. After the abdication of Diocletian, the persecution continued in the East under Galerius until 311, but was abandoned in the West by 306.
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On 1 May 305, in simultaneous ceremonies in Nicomedia and Mediolanum, Diocletian and Maximian both retired, setting a precedent in the hopes of stabilizing the office of Emperor and preventing further succession crises. Constantius Chlorus and Galerius became the new Augusti, but remained based in Gaul and the Balkans, respectively. Although Constantius was technically the senior Augustus, Galerius was Diocletian’s informal successor and appointed his own officers, Valerius Severus and Maximinus Daza, as Caesars.
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Following Constantius Chlorus’ promotion to Augustus in May 305, his eldest son Constantine was released from the court of Galerius and raced west, meeting Constantius at Bononia (Boulogne) as he was about to depart for Britain. The two campaigned against the Picts together, when Constantius fell ill, dying at Eboracum (York) on 25 July 306. The troops immediately rallied behind the 34-year-old Constantine, proclaiming him emperor in his father’s place.
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When Galerius learned of Constantius Chlorus’ death, he promoted Valerius Severus to Augustus in the West but initially refused to acknowledge Constantine’s accession to power in Gaul. However, after it became clear that the Gallic legions firmly supported Constantine, Galerius backed down and acknowledged him as Caesar, but emphasized Severus’ seniority by giving the latter authority over the dioceses of Hispaniae and Pannoniae.
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In 306 Galerius, as senior Augustus, attempted to end Rome’s tax exempt status and reduce the size of the Praetorian Guard, provoking dissent in what was still the imperial capital. After killing several officials, the Praetorians found Maxentius, the twenty-something son of the former Augustus Maximian and son-in-law of Galerius, at his villa and proclaimed him emperor. Maxentius, bitter at being passed over for leadership earlier, accepted the title of Princeps, possibly hoping that Galerius would acknowledge him as had happened with Constantine.
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Maxentius’ coup in Rome quickly gained support across Italy south of the Po, as well as the islands of Corsica, Sardinia, and Sicily. When news of the revolt reached Valerius Alexander in Carthage, he too immediately declared for Maxentius—it is possible that the government of Africa still retained loyalty to Maxentius’ father Maximian, who had successfully campaigned there in 297–8.
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