Placidia spent most of her early years in the household of Stilicho the Vandal and his wife Serena. She is presumed to have learned weaving and embroidery. She might have also been given a classical education though no details are known. Serena was a first cousin of Arcadius, Honorius and Placidia. The poem "In Praise of Serena" by Claudian and the "Historia Nova" by Zosimus clarify that Serena's father was an elder Honorius, a brother to Theodosius I. According to "De Consulatu Stilichonis" by Claudian, Placidia was betrothed to Eucherius, only known son of Stilicho and Serena. Her scheduled marriage is mentioned in the text as the third union between Stilicho's family and the Theodosian dynasty, following those of Stilicho to Serena and Maria, their daughter, to Honorius.
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. He was also titled "magister equitum et peditum" ("Master of the Horse and of Foot"), placing him in charge of both the cavalry and infantry forces of the Western 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, "an officer of rank in the court-guards" 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 by Arsacius and Tarentius, two eunuchs following imperial command. They executed him not long after. Honorius appointed Tarentius imperial chamberlain, and gave the next post under him to Arsacius. Their deaths left Placidia effectively unattached.
Prior to the fall of Rome, Placidia was captured by Alaric. Her captivity was recorded by both Jordanes and Marcellinus Comes, though the exact circumstances are not mentioned. 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. The nuptials were celebrated with high Roman festivities and magnificent gifts from the Gothic booty. Priscus Attalus gave the wedding speech, a classical epithalamium. The marriage was recorded by Hydatius. The historian Jordanes states that they married earlier, in 411 at Forum Livii (Forlì). Jordanes's date may actually be when she and the Gothic king first became more than captor and captive.
Placidia and Ataulf had a single known son, Theodosius. He was born in Barcelona by the end of 414. Theodosius died early in the following year, thus eliminating an opportunity for a Romano-Visigothic line. Years later the corpse was exhumed and reburied in the imperial mausoleum in St. Peter's Basilica, Rome. In Hispania, Ataulf imprudently accepted into his service a man identified as "Dubius" or "Eberwolf", a former follower of Sarus. Sarus was a Germanic chieftain who was killed while fighting under Jovinus and Sebastianus. His follower harbored a secret desire to avenge the death of his beloved patron. And so, in the palace at Barcelona, the man brought Ataulf's reign to a sudden end by killing him while he bathed in August/September, 415.
The Amali faction proceeded to proclaim Sigeric, a brother of Sarus, as the next king of the Visigoths. According to the The History of the Decline and Fall of the Roman Empire by Edward Gibbon, the first act of Sigeric's reign "was the inhuman murder" of Ataulf's six children from a former marriage "whom he tore, without pity, from the feeble arms of a venerable bishop." (the later being Sigesar, Bishop of the Goths). As for Galla Placidia, as Ataulf's widow, she was "treated with cruel and wanton insult" by being forced to walk more than twelve miles on foot among the crowd of captives driven ahead of the mounted Sigeric. Seeing the noble widow's sufferings, however, became one of the factors that roused indignant opponents of the usurper, who quickly assassinated Sigeric and replaced him with Wallia, Ataulf's relative.
According to the Chronicon Albeldense, included in the Roda Codex, Wallia was in desperate need of food supplies. He surrendered to Constantius III, at the time magister militum of Honorius, negotiating terms giving foederati status for the Visigoths. Placidia was returned to Honorius as part of the 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. The history of Paul the Deacon mentions her first when mentioning the children of the marriage, suggesting she was the eldest. Their son Valentinian III was born on 2 July, 419.
Placidia intervened in the succession crisis following the death of Pope Zosimus on 26 December, 418. Two factions of the Roman clergy had proceeded to elect their own Popes, the first electing Eulalius (27 December) and the other one electing Boniface I (28 December). They acted as rival Popes while situated in the same city, Rome. Said city was thrown into tumult as both factions clashed. Symmachus, Prefect of Rome, sent his report to the imperial court at Ravenna, requesting an imperial decision on the matter. Placidia and ,presumably, Constantius petitioned the emperor in favor of Eulalius. This was arguably the first intervention by an Emperor in the Papal election.
Honorius initially confirmed Eulalius as the legitimate Pope. As this failed to put an end to the controversy, Honorius called a Synod of Italian bishops at Ravenna to decide on the matter. The Synod convened from February to March 419 but failed to reach a conclusion. Honorius decided to call a second Synod in May, this time calling Gaulish and African bishops to participate. In the mean time, the two rival Popes were ordered to leave Rome. However as Easter approached, Eulalius returned to the city and attempted to seize the Basilica of St. John Lateran in order to "preside at the paschal ceremonies". Imperial troops managed to repell him and on Easter (30 March, 419) the ceremonies were presided by Achilleus, Bishop of Spoleto. The conflict cost Eulalius' imperial favor and Boniface was proclaimed the legitimate Pope as of 3 April, 419, returning to Rome a week later. Placidia had personally written to the African bishops, summoning them to the second synod. Three of her letters are known to have survived.
On 8 February, 421, Constantius was proclaimed an Augustus, becoming a co-ruler with the childless Honorius. Placidia was proclaimed an Augusta. She was the only Empress in the West, since Honorius had divorced Thermantia, his second wife, in 408 and never remarried. Both titles were not recognized by Theodosius II, 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 2 September, 421.
Galla herself, the former Augusta, was however forced from the Western Empire. Whatever the politics or motivations, the public issue was increasingly scandalous public sexual caresses from her own brother Honorius. This at least was the interpretation given by Olympiodorus of Thebes, a historian used as a source by Zosimus, Sozomen and probably Philostorgius, as J.F. Matthews has demonstrated. 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." Gibbon compares the public caresses to those of Muhammad to his daughter Fatimah, as a "sensual indulgence" without actual incest involved.
According to Gibbon, "On a sudden, by some base intrigues of a steward and a nurse, this excessive fondness was converted into an irreconcilable quarrel: the debates of the emperor and his sister were not long confined within the walls of the palace; and as the Gothic soldiers adhered to their queen, the city of Ravenna was agitated with bloody and dangerous tumults, which could only be appeased by the forced or voluntary retreat of Placidia and her children. The royal exiles landed at Constantinople, soon after the marriage of Theodosius, during the festival of the Persian victories. They were treated with kindness and magnificence; but as the statues of the emperor Constantius had been rejected by the Eastern court, the title of Augusta could not decently be allowed to his widow". The passage places the arrival of Placidia and her children as following the marriage of Theodosius II to Aelia Eudocia, known to have occurred on 7 June, 421. The "Persian victories" mentioned were probably victory celebrations over a brief Roman-Persian War, under the respective leaderships of Theodosius II and Bahram V of the Sassanid Empire. The conflict took place from c. 420 to 422. "The general Ardaburius operated in Arzanene and gained a victory, autumn 421, which forced the Persians to retreat to Nisibis, which Ardaburius then besieged. He raised the siege on the arrival of an army under Varahran, who proceeded to attack Resaina. Meanwhile the Saracens of Hira, under Al‑Mundhir, were sent to invade Syria, and were defeated by Vitianus. During the peace negotiations the Persians attacked the Romans and were defeated by Procopius, son-in‑law of Anthemius (Socrates, VII.18, 20). The Empress Eudocia celebrated the war in a poem in heroic metre (ib. 21). The "Saracens of Hira" were the Lakhmids of Al-Hirah.
On 15 August, 423, Honorius died of dropsy, perhaps pulmonary edema. With no member of the Theodosian dynasty present at Ravenna to claim the throne, Theodosius II was expected to nominate a Western co-emperor. However Theodosius hesitated and the decision was delayed. Taking advantage of the power vacuum, Castinus the Patrician proceeded to become a kingmaker. He declared Joannes, the "primicerius notariorum" ("chief notary", head of the civil service), to be the new Western Roman Emperor. Among their supporters was Flavius Aetius. Aetius was a son of Flavius Gaudentius, magister militum, and Aurelia. Joannes' rule was accepted in the provinces of Italia, Gaul, Hispania, but not in Africa Province.
Theodosius II reacted by starting to prepare Valentinian III for eventual promotion to the imperial office. Within 423/424, Valentinian was named "nobilissimus". In 424, Valentinian was betrothed to Licinia Eudoxia, his first cousin, once removed. She was a daughter of Theodosius II and Aelia Eudocia. The year of their betrothal was recorded by Marcellinus Comes. At the time of their betrothal, Valentinian was approximately four-years-old, Licinia only two. Gibbon attributes the betrothal to "the agreement of the three females who governed the Roman world", meaning Placidia and her nieces Eudocia and Pulcheria. Within the same year, Valentinian was proclaimed a Caesar in the Eastern court.
The campaign against Joannes also started within the same year. Forces of the Byzantine army gathered at Thessaloniki, there placed under the general command of Ardaburus, the victorious general of the Roman-Persian War. The invasion force was to cross the Adriatic Sea through two different trails. Aspar, son of Ardaburius, was entrusted to lead the cavalry through land, following the sea-coast of the Adriatic from the Western Balkans to Northern Italy. Placidia and Valentinian joined Aspar's force. Ardaburius and the infantry boarded ships of the Byzantine navy in an attempt to reach Ravenna by sea. Aspar marched his forces to Aquileia, taking the city by surprise and with virtual no resistance. On the other hand, the fleet was dispersed by a storm. Ardaburius and two of his galleys were captured by forces loyal to Joannes, held prisoners in Ravenna.
Ardaburius was treated well by Joannes, who probably intended to negotiate with Theodosius for an end to the hostilities. The prisoner was allowed the "courteous freedom" of walking the court and streets of Ravenna while his captivity lasted. He took advantage of this privilege to come into contact with the forces of Joannes, convince some of them to defect to Theodosius' side. The conspirators contacted Aspar and beckoned him to Ravenna. Aspar had a shepherd lead his cavalry force through the marshes of Po River to the gates of Ravenna. With the besiegers outside the walls and the defectors within, the city was captured with little further struggle. Joannes was captured with his capital, his right hand cut off. He was then mounted to a donkey, paraded through the streets. He was finally executed by decapitation in the Hippodrome of Aquileia.
With Joannes dead, Valentinian was officially proclaimed the new Augustus of the Western Roman Empire on 23 October, 425. His proclamation took place in the presence of the Roman Senate. Three days following Joannes' death, Aetius brought reinforcements for his army, a reported number of sixty thousand Huns from across the Danube. After some skirmishing, Placidia and Aëtius came to an agreement that established the political landscape of the Western Roman Empire for the next thirty years. The Huns were paid off and sent home, while Aetius received the position of magister militum per Gallias(commander-in-chief of the Roman army in Gaul).
Galla was the regent of the Western Roman Empire from 425 to 437, her regency ending when Valentinian reached his eighteenth birthday on 2 July, 437. Among her early supporters was Bonifacius, governor of the Diocese of Africa. Aetius was arguably his rival for influence, managing 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.
Conflict between Placidia and Bonifacius started in 429. According to "History of the Wars" by Procopius:"There were two Roman generals, Aetius and Boniface, especially valiant men and in experience of many wars inferior to none of that time at least. These two came to be at variance in regard to matters of state, but they attained to such a degree of highmindedness and excellence in every respect that if one should call either of them "the last of the Romans" he would not err, so true was it that all the excellent qualities of the Romans were summed up in these two men. One of these, Boniface, was appointed by Placidia general of all Libya. Now this was not in accord with the wishes of Aetius, but he by no means disclosed the fact that it did not please him. For their hostility had not as yet come to light, but was concealed behind the countenance of each. But when Boniface had got out of the way, Aetius slandered him to Placidia, saying that he was setting up a tyranny and had robbed her and the emperor of all Libya, and he said that it was very easy for her to find out the truth; for if she should summon Boniface to Rome, he would never come. And when the woman heard this, Aetius seemed to her to speak well and she acted accordingly.
"But Aetius, anticipating her, wrote to Boniface secretly that the mother of the emperor was plotting against him and wished to put him out of the way. And he predicted to him that there would be convincing proof of the plot; for he would be summoned very shortly for no reason at all. Such was the announcement of the letter. And Boniface did not disregard the message, for as soon as those arrived who were summoning him to the emperor, he refused to give heed to the emperor and his mother, disclosing to no one the warning of Aetius. So when Placidia heard this, she thought that Aetius was exceedingly well-disposed towards the emperor's cause and took under consideration the question of Boniface."
"But Boniface, since it did not seem to him that he was able to array himself against the emperor, and since if he returned to Rome there was clearly no safety for him, began to lay plans so that, if possible, he might have a defensive alliance with the Vandals, who, as previously stated, had established themselves in Spain, not far from Libya. There Godigisclus had died and the royal power had fallen to his sons, Gontharis, who was born to him from his wedded wife, and Gizeric,of illegitimate birth. But the former was still a child and not of very energetic temper, while Gizeric had been excellently trained in warfare, and was the cleverest of all men. Boniface accordingly sent to Spain those who were his own most intimate friends and gained the adherence of each of the sons of Godigisclus on terms of complete equality, it being agreed that each one of the three, holding a third part of Libya, should rule over his own subjects; but if a foe should come against any one of them to make war, that they should in common ward off the aggressors.
"On the basis of this agreement the Vandals crossed the strait at Gadira and came into Libya, and the Visigoths in later times settled in Spain. But in Rome the friends of Boniface, remembering the character of the man and considering how strange his action was, were greatly astonished to think that Boniface was setting up a tyranny, and some of them at the order of Placidia went to Carthage. There they met Boniface, and saw the letter of Aetius, and after hearing the whole story they returned to Rome as quickly as they could and reported to Placidia how Boniface stood in relation to her. And though the woman was dumbfounded, she did nothing unpleasant to Aetius nor did she upbraid him for what he had done to the emperor's house, for he himself wielded great power and the affairs of the empire were already in an evil plight; but she disclosed to the friends of Boniface the advice Aetius had given, and, offering oaths and pledges of safety, entreated them to persuade the man, if they could, to return to his fatherland and not to permit the empire of the Romans to lie under the hand of barbarians.
"And when Boniface heard this, he repented of his act and of his agreement with the barbarians, and he besought them incessantly, promising them everything, to remove from Libya. But since they did not receive his words with favour, but considered that they were being insulted, he was compelled to fight with them, and being defeated in the battle, he retired to Hippo Regius, a strong city in the portion of Numidia that is on the sea. There the Vandals made camp under the leadership of Gizeric and began a siege; for Gontharis had already died. And they say that he perished at the hand of his brother. The Vandals, however, do not agree with those who make this statement, but say that Gontharis' was captured in battle by Germans in Spain and impaled, and that Gizeric was already sole ruler when he led the Vandals into Libya. This, indeed, I have heard from the Vandals, stated in this way. But after much time had passed by, since they were unable to secure Hippo Regius either by force or by surrender, and since at the same time they were being pressed by hunger, they raised the siege. And a little later Boniface and the Romans in Libya, since a numerous army had come from both Rome and Byzantium and Aspar with them as general, decided to renew the struggle, and a fierce battle was fought in which they were badly beaten by the enemy, and they made haste to flee as each one could. And Aspar betook himself homeward, and Boniface, coming before Placidia, acquitted himself of the suspicion, showing that it had arisen against him for no true cause.
Bonifacius and Aetius initiated direct conflict against each other in 432. According to Gibbon: "Boniface accepted with gratitude the rank of patrician, and the dignity of master-general of the Roman armies; but he must have blushed at the sight of those medals, in which he was represented with the name and attributes of victory. The discovery of his fraud, the displeasure of the empress, and the distinguished favor of his rival, exasperated the haughty and perfidious soul of Aetius. He hastily returned from Gaul to Italy, with a retinue, or rather with an army, of Barbarian followers; and such was the weakness of the government, that the two generals decided their private quarrel in a bloody battle. Boniface was successful; but he received in the conflict a mortal wound from the spear of his adversary, of which he expired within a few days, in such Christian and charitable sentiments, that he exhorted his wife, a rich heiress of Spain, to accept Aetius for her second husband. But Aetius could not derive any immediate advantage from the generosity of his dying enemy: he was proclaimed a rebel by the justice of Placidia; and though he attempted to defend some strong fortresses, erected on his patrimonial estate, the Imperial power soon compelled him to retire into Pannonia, to the tents of his faithful Huns. The republic was deprived, by their mutual discord, of the service of her two most illustrious champions."
With the generals loyal to having either died or defected to Aëtius, Placidia apparently succumbed to the inevitable. Aetius was recalled from exile in 433, given the titles of "magister militum" and "Patrician". The appointments effectively left Aetius in control of the entire Western Roman Army and with considerable influence over imperial policy. She continued to act as regent until 437, though her direct influence over decisions had diminished. She would continue to exercise political influence to her death in 450, no longer however the only power at court.
Aetius, was later pivotal to the defense of the Western Empire against Attila the Hun. Attila was diverted from his focus on Constantinople towards Italy as his target due to a letter from Placidia's own daughter Justa Grata Honoria in the spring of 450, asking him to rescue her from an unwanted marriage to a Roman senator that the Imperial family, including Placidia, was trying to force on her. Honoria had also sent her engagement ring with the letter. Though Honoria may not have intended a proposal of marriage, Attila chose to interpret her message as such. He accepted, asking for half of the western Empire as dowry. When Valentinian discovered the plan, only the influence of Placidia convinced him to exile, rather than kill, Honoria. He also wrote to Attila strenuously denying the legitimacy of the supposed marriage proposal. Attila, not convinced, sent an emissary to Ravenna to proclaim that Honoria was innocent, that the proposal had been legitimate, and that he would come to claim what was rightfully his. Honoria was married to Flavius Bassus Herculanus, though this did not prevent Attila to still press his claim
Placidia died shortly afterwards at Rome in November 450, and did not live to see Attila ravage Italy in 451 – 453 in a much more brutal campaign than the Goths had waged, using Justa's letter as their sole "legitimate" excuse.
Placidia was a fervant Chalcedonian Christian. She was involved in the building and restoration of various churches throughout her period of influence. She restored and somewhat expanded the Basilica of Saint Paul Outside the Walls in Rome and the Church of the Holy Sepulchre in Jerusalem. She built San Giovanni Evangelista, Ravenna, the later in thanks for the sparing of her life and those of her children in a storm while crossing the Adriatic Sea. The dedicatory inscription reads "Galla Placidia, along with her son Placidus Valentinian Augustus and her daughter Justa Grata Honoria Augusta, paid off their vow for their liberation from the danger of the sea.
Her Mausoleum in Ravenna was one of the UNESCO World Heritage Site inscribed in 1996. However there is some doubt whether the building served as her tomb. The building was initially erected as a chapel dedicated to Lawrence of Rome. Whether the sarcophagi contained within contained the corpses of the Theodosian dynasty or not, the time of their entry in the building is unknown.