[4] Quarrels over the sharing of power led to a civil war with his eldest brother and co-emperor Constantine II, who invaded Italy in 340 and was killed in battle by Constans's forces near Aquileia.

[4] Thereafter there were tensions with his remaining brother and co-augustus Constantius II (r. 337–361), including over the exiled bishop Athanasius of Alexandria,[4] who in turn eulogized Constans as "the most pious Augustus... of blessed and everlasting memory.

[6] In January 350, Magnentius (r. 350–353) the commander of the Jovians and Herculians, a corps in the Roman army, was acclaimed augustus at Augustodunum (Autun) with the support of Marcellinus, the comes rei privatae.

[18] Although the reasons for the visit remain unclear,[20] the ancient writers were primarily interested in Constans' precarious journey to the province, rather than his actions within it.

[18] One theory considers it to have involved the northern frontier, based on Ammianus' remark that he had discussed the Areani in his now-lost coverage of Constans' reign.

Additionally, after recording attacks "near the frontiers" in 360, the historian wrote that the Alamanni were too much of a threat for Julian to confront the problem, in contrast to what Constans was able to do.

[24] Despite Eugenius being alleged to have misused his power to seize property,[24] the emperor continued to support him, his trust going as far as to honor him with a statue in the Forum of Trajan in Rome.

Although the two emperors called the Council of Serdica in 343 to settle the conflict, it was a complete failure,[29] and by 345 Constans was outright threatening civil war against his brother.

[34][35] On 18 January 350,[36] the general Magnentius declared himself emperor at Augustodunum (Autun) with the support of a number of court officials such as Marcellinus, Constans' comes rerum privatarum, as well as Fabius Titianus, who had previously served as the praetorian prefect of Gaul.

[37] As he was trying to reach Hispania, supporters of Magnentius cornered him in a fortification in Helena (Elne) in the eastern Pyrenees of southwestern Gaul, where he was killed after seeking sanctuary in a temple.

[22][15] Ammianus lamented the emperor's failure to listen to wise counsel,[34] referencing one man he believed could have saved Constans from his own faults.

[47] On speculating the basis for Constans' overthrow, she suggested that one reason may have been regarding financial difficulties in Gaul by the end of his reign, which could have been related to the finance officer Marcellinus' support of him.