Placidia was the daughter of Theodosius I and his second wife, Galla, who was herself daughter of Valentinian I and his second wife, Justina.
She was regent to Valentinian III from 423 until his majority in 437, and a major force in Roman politics for most of her life. She was queen consort to Ataulf, king of the Visigoths from 414 until his death in 415, and briefly empress consort to Constantius III in 421. Coins issued in Placidia’s honour in Constantinople after 425 give her name as AELIA PLACIDIA; this may have been intended to integrate Placidia with the eastern dynasty of Theodosius II. There is no evidence that the name Aelia was ever used in the west, or that it formed part of Placidia’s official nomenclature.
Placidia was granted her own household by her father in the early 390s and was thus financially independent while underage. She was summoned to the court of her father in Mediolanum (Milan) during 394, and was present at Theodosius‘ death on January 17, 395. She was granted the title of “nobilissima puella” (“most noble girl”) during her childhood. Placidia spent most of her early years in the household of Stilicho and his wife, Serena. Stilicho was the magister militum of the Western Roman Empire. He was the only known person to hold the rank of “magister militum in praesenti” from 394 to 408 in both the Western and the Eastern Roman Empire. In 408, Arcadius died and was succeeded by his son Theodosius II, only seven years old. Stilicho planned to proceed to Constantinople and “undertake the management of the affairs of Theodosius”, convincing Honorius not to travel to the East himself. Shortly after, Olympius, ‘Magister Scrinii‘, attempted to convince Honorius that Stilicho was in fact conspiring to depose Theodosius II, to replace him with Eucherius. Olympius proceeded to lead a military coup d’état which left him in control of Honorius and his court. Stilicho was arrested and executed on August 22, 408. Eucherius sought refuge in Rome but was arrested there and executed.
In the disturbances that followed the fall of Stilicho, the wives and children of the foederati living in the cities of Italy were slain. Most of the foederati, who were considered loyalists of Stilicho, joined the forces of Alaric I, King of the Visigoths. Alaric led them to Rome and began a blockade of the city, which was under siege, with minor interruptions, from autumn 408 to August 24, 410. Zosimus records that Placidia was within the city during the siege. When Serena was accused of conspiring with Alaric, “the whole senate therefore, with Placidia, uterine sister to the emperor, thought it proper that she should suffer death”.
Prior to the fall of Rome, Placidia was captured by Alaric. She followed the Visigoths in their move from the Italian Peninsula to Gaul in 412. Their ruler Ataulf, having succeeded Alaric, entered an alliance with Honorius against Jovinus and Sebastianus, rival Western Roman emperors located in Gaul. He managed to defeat and execute both Gallo-Roman emperors in 413. After the heads of Sebastianus and Jovinus arrived at Honorius’ court in Ravenna in late August, to be forwarded for display among other usurpers on the walls of Carthage, relations between Ataulf and Honorius improved sufficiently for Ataulf to cement them by marrying Galla Placidia at Narbonne on January 1, 414. Placidia and Ataulf had a single known son, Theodosius. He was born in Barcelona by the end of 414. In August/September, 415, in the palace at Barcelona, a former Germanic chieftain, follower of Jovinus and Sebastianus brought Ataulf’s reign to a sudden end by killing him while he bathed.
Placidia was returned to Honorius as part of a peace treaty. Her brother Honorius forced her into marriage to Constantius III on January 1, 417. Their daughter Justa Grata Honoria was probably born in 417 or 418. Their son Valentinian III was born July 2, 419.
On February 8, 421, Constantius was proclaimed an Augustus, becoming co-ruler with the childless Honorius. Placidia was proclaimed an Augusta. She was the only Empress in the West, since Honorius had divorced his second wife Thermantia in 408 and had never remarried. Neither title was recognised by Theodosius II, the Eastern Roman Emperor. Constantius reportedly complained about the loss of personal freedom and privacy that came with the imperial office. He died of an illness on September 2, 421.
Galla Placidia herself was now forced from the Western Empire. Though the motivation for this remains unclear, the public issue was the increasingly scandalous public caresses she received from her own brother Honorius—this at least was the interpretation of Olympiodorus of Thebes, a historian used as a source by Zosimus, Sozomen and probably Philostorgius. Gibbon had a different opinion: “The power of Placidia; and the indecent familiarity of her brother, which might be no more than the symptoms of a childish affection, were universally attributed to incestuous love.”
Galla Placidia was regent of the Western Roman Empire from 425 to 437, her regency ending when Valentinian reached his eighteenth birthday on July 2, 437. Among her early supporters was Bonifacius, governor of the Diocese of Africa. Aetius, his rival for influence, managed to secure Arles against Theodoric I of the Visigoths. The Visigoths concluded a treaty and were given Gallic noblemen as hostages. The later Emperor Avitus visited Theodoric, lived at his court and taught his sons.