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Gerontius (fl. 407–411) was a general of the Western Roman Empire (with the rank of magister militum),^[1] who initially supported the usurper Constantine III, later betraying him in favour of another usurper, Maximus of Hispania.

Gerontius, probably a Briton by birth, was one of the supporters of Constantine III, a Roman general who revolted against the emperor Honorius in 407, conquering Britain, Gaul, Germania, and Hispania.^[2]

Constantine's control of Hispania was challenged by the revolts of Didymus and Verenianus, cousins of Honorius. In early 408, Constantine sent an army against the rebels, headed by his son Constans and commanded by Gerontius. After losing his first battle against the rebels, Gerontius recalled additional forces from Gaul and won an important victory in Lusitania, where Didymus and Verenianus were captured. Constans escorted the prisoners back to Gaul, while Gerontius remained in Hispania, in command of the local troops.^[3]

In late spring 409, Gerontius rebelled against Constantine, who was now officially recognized as co-emperor by Honorius, and appointed Maximus as emperor.^[4] He moved to Tarraco, but left the city to oppose an invasion of Hispania by Constans, whom Constantine had newly appointed Augustus. Gerontius's motivations are unclear; the close timing of the revolt and of Constans's elevation to the throne suggests that one motivated the other, but it is not known which event came first.

To counteract Constantine, Gerontius allied with Vandals, Alans, and Suebi.^[5] A Frankish revolt occupied Constantine, who was forced to reduce his pressure on Gerontius, and the Franks and their allies entered Hispania, where they became a problem for Gerontius.

Gerontius successfully defended himself against Constans during 410. In 411, he successfully besieged and captured Constans in Vienne, executing him.^[6] He then moved on Arelate, where he besieged Constantine. However, while Constantine was still resisting, Honorius's general Constantius arrived from Italy with an army. Most of Gerontius's soldiers deserted and he was obliged to flee back to Hispania with a few loyal supporters.^[7]

According to Sozomen, the remaining troops turned on Gerontius once they knew of Gerontius's defeat, and blocked Gerontius and his wife into a house. Gerontius attempted to keep the aggressors away by shooting arrows at them from the roof. When his arrows ran out, the soldiers set fire to the house. Hopelessly defeated, he killed his wife before committing suicide.^[8]

Gerontius' reliance on Vandals, Alans, and Suebi allies is considered the beginning of barbarian invasions in Hispania.^[9] Furthermore, the campaign led by Constantine III initially with troops from Roman Britain, including Gerontius, and the chaos brought to Gaul "forced the inhabitants of Britain and some of the tribes in Gaul to secede from Roman rule and to be independent, obeying Roman laws no longer."^[10]

• Kulikowski, Michael, Late Roman Spain and Its Cities, Johns Hopkins University Press, 2004, ISBN 0-8018-7978-7, p. 157-160.