This article chronicles the attested movements of the fourth-century Roman emperors Constantine II (referred to here as Constantinus), Constantius II (referred to here as Constantius), Constans, Gallus, and Julian the Apostate from 337 to 361 AD. It does not cover the imperial usurpers of the period, including Magnentius, Vetranio, Claudius Silvanus, and Poemenius.^[1] The chronology is principally derived from Timothy Barnes' Athanasius and Constantius.^[2] Substantial additions^[which?] and further sources^[which?] are based on recent research^{[which?][by whom?]} that seeks to go beyond Barnes' own chronology and slightly modifying his at a few places.^[which?]

This article possibly contains original research. (August 2021)

Constantinian family tree, showing Constantius I and his children

Constantinian family tree, showing Constantine I and his children

This article begins its coverage at the death of Constantine on 22 May 337. After an interregnum of three months, during or after which the army and its agents lynched other potential successors, the three sons of Constantine declared themselves Augusti on 9 September 337.^[3] Discarding their father's succession arrangements, the brothers divided the empire into three parts.^{[4][notes 1]} Constantine II ruled the provinces of Gaul, Britain, Spain, and Germany from Trier.^{[6][notes 2]} Constantius ruled the provinces of Asia Minor (the dioceses of Pontus and Asia), Thrace, the Levant and Egypt (the Diocese of the East) from Antioch.^[8] Constans ruled Italy, Africa, and the dioceses of Pannonia, Dacia, and Macedonia from Naissus.^[9]

In 340, Constantinus attempted to seize his brother Constans' territory, and was killed while reconnoitering Ravenna during the ensuing civil war. Constans acquired his territory, and ruled from Trier, Milan, and Sirmium.^[10] In January 350, Constans was overthrown at Autun and killed at Elne in a palace revolution instigated by Magnentius. Magnentius was defeated and killed in the summer of 353 at the Battle of Mons Seleucus, making Constantius the sole emperor.^[11]

From 351 to 359, Constantius ruled from Sirmium and Milan, and from Constantinople and Antioch.^[12] Constantius appointed Gallus Caesar (subordinate emperor) on 15 March 351, and delegated the rule of the eastern provinces to him. Gallus ruled from Antioch.^[13] He subsequently proved violent and cruel, and was recalled and executed in autumn 354.^[14] Constantius appointed Julian, the last surviving male relative of Constantine other than himself, Caesar on 6 November 355.^[15] Julian ruled the western provinces from Vienne, Sens, and Paris.^[13] To address Persian influence and aggression on the eastern frontier, Constantius ruled from Antioch from 360 until his death.^[12]

Julian's troops proclaimed him Augustus in February 360. Constantius did not recognize Julian's claim to the title, but was detained from campaigning against him by Persian raids. Constantius died on 3 November 361 after declaring Julian his successor.^[16] Julian was sole emperor from Constantius' death until his own death in 363.^[17] This article ends its coverage at Julian's death on 26 June 363. For a further timeline until 426, consult Matthews, John. Western Aristocracies and Imperial Court A.D. 364–425. Oxford: Clarendon Press; New York: Oxford University Press, 1975.

A † indicates that a date or an event is uncertain. A superscript S indicates that the manuscript is corrupt, and has been emended to follow Otto Seeck's corrections in his edition of the Codex Theodosianus.^[18] Manuscript details are given in brackets (as "mss. date" or "mss. year", etc.) for all emended texts. Unsourced events are purely conjectural. Note that some are based on triangulation of different sources and can be, as such, somewhat inexact. These cases have been noted.