Constantius III Flavius Constantius ( died 2 September 421 ) , commonly known as Constantius III , was Western Roman Emperor for seven months in 421 . A prominent general and politician , he was the power behind the throne for much of the 410s , and in 421 briefly became co - emperor of the Western Empire with Honorius . Life . Early life and rise to the throne . Constantius was born in Naissus ( modern - day Niš , Serbia ) and was probably a career soldier , who reached the rank of " magister militum " under Honorius . In 411 he defeated Gerontius , the general of usurper Maximus , and Constantine III , another usurper , in Arles . Constantine had revolted against Honorius ( 407 ) , but then he had broken his relationship with general Gerontius , who had proclaimed Maximus Emperor in opposition to both Honorius and Constantine ( 409 ) . Gerontius had then besieged Constantine in the city of Arles ( 411 ) , but the arrival of the troops of Constantius from Italy had put him in serious trouble , being at the same time a besieger and a besieged . Gerontius was abandoned by his troops and then killed in Hispania . The able general having been disposed of , Constantius besieged the city for three months until the general of Constantine , Edobichus , who had been sent across the border to find allies , did return with a large army of Franks and Alamanni . Before the walls of Arles , Constantius confronted and defeated Edobichus , who was later betrayed and killed by a friend . Constantine was forced to surrender to Constantius when his troops on the Rhine left him for another usurper , Jovinus . Constantius granted a safe conduct to Constantine , who had become an ordained priest , but later he captured and killed him . In 412 he expelled from Italy the Visigoths under Ataulf . In 413 he held his first consulate . In 414 he began a military offensive against the Visigoths . In response , Ataulf appointed as emperor Priscus Attalus , who had already been a usurper against Honorius in 410 . The blockade imposed by Constantius on Gallic ports was so effective that the Visigoths fled from Gaul to Hispania in 415 . Attalus too tried to flee but was captured by the forces of Constantius and sent to Ravenna . With the death of Ataulf and his successor Segeric , that same year Constantius signed a treaty with the new Visigothic king Wallia : in exchange of 600 , 000 bushels of wheat and the territory of the region of Aquitaine , from the Pyrenees to the Garonne , the Visigoths pledged to fight on behalf of the Romans , as allies official or vassal state of the Empire ( " foederati " ) , the Vandals , Alans and Suebi who in 407 had crossed the Rhine River and were stationed in the provinces of Hispania . The agreement also provided for the release of Galla Placidia , Honorius ' sister , captured during the sack of Rome in 410 . Constantius was appointed " patricius " and in 417 , the year of his second consulate held with Honorius as colleague , he married Galla Placidia , tying himself to the House of Theodosius . The couple had two children , Justa Grata Honoria ( born 417 / 418 ) and the future Emperor Valentinian III ( born 419 ) . In 420 he was appointed consul for the third time , with the Eastern Emperor Theodosius II as colleague . Brief reign and death . On February 8 , 421 , Constantius was appointed co - emperor with his ineffectual brother - in - law , Honorius , becoming the real master of the West . Curiously , he complained of loss of personal freedom implied by the new role . The elevation of Constantius , however , was not recognized by his colleague in the East , Theodosius II , who was the nephew of Honorius . It is said that Constantius was organizing a military expedition to the East to have his rights recognized , when he died suddenly on September 2 , 421 , after just seven months as emperor , the archetype of able soldier and politician that the Western Roman Empire desperately needed at the time . " A struggle to replace Constantius swiftly erupted , with some open fighting . Galla Placidia fled with her son Valentinian to Constantinople . Her brother Honorius , still childless , died in 423 . " Constantius ' success in rising from head of the Roman army to Imperial rank obviously influenced the actions of later holders of the patrician office , a list that includes Flavius Aëtius and Ricimer ; however , only Petronius Maximus ever made the same leap , and his reign was even shorter than that of Constantius .