Valentinian III was born in AD 419, the son of Constantius III and Aelia Galla Placidia, who was the half-sister of Honorius.
With Honorius having no children of his own, he proclaimed Valentinian III his heir (AD 421 or shortly after).
Honorius died later in AD 423 and, with the heir apparent absent, his throne was usurped by John, a civil servant who was manoeuvered into position by Castinus, the powerful 'Master of Soldiers' of the west, who was an enemy of Placidia.
Theodosius II, opposed to the usurper, now belatedly recognized Constantius III (who had died in AD 421) as Augustus, in order to champion the cause of Valentinian III.
The two generals Ardaburius and Aspar succeeded in conquering Ravenna and John was captured, sentenced to death by Placidia in Aquileia and executed in May or June AD 425.
Valentinian III was hailed Augustus in Rome in AD 425 at the age of six. Placidia became the regent of the western empire.
Valentinian III, like other emperors of that period (for example Honorius and Theodosius II) was a ruler, who left the governing of his empire to others. At first, during his childhood, he should have his mother Placidia to take care of government and later rule lay much in the hands of his military chief Aetius. While others governed Valentinian III chose far more to have a good time. He enjoyed physical activity, was said to have been a good runner, and a skilled horseman and archer. Further he liked the company of astrologers and magicians.
The first crisis in Valentinian III's reign was soon to come, when general Aetius arrived with his relief force of Huns only to find that emperor John, whom he sought to save, had already been executed.
In the same year AD 429 the Vandal leader Geiseric crossed from Spain to north Africa with his people. There he declared himself an independent king, free from Rome or any allegiance to Valentinian III, and set out to conquer Rome's African domains.
In AD 431 Geiseric's Vandals moving east heavily defeated the Romans in north Africa.
Meanwhile back in Europe Placidia hoped for her main army supporter, Bonifatius (Boniface), to rid her of Aetius. Bonifatius had returned from Africa to Italy in AD 429 and ever since war between him and Aetius had been brewing. The two finally clashed in AD 432. Bonifatius was defeated, wounded and later died.
Aetius went on to drive the Germans back behind the Upper Danube, put down revolts in Gaul and in AD 437 he smashed the Burgundians as they sought to invade Gaul. Considering the military difficulties of the time and the shortage of soldiers in the Roman army, these victories were outstanding achievements.
Yet the deterioration in north Africa continued. In AD 439 Geiseric conquered Carthage and in AD 441 once again heavily defeated a Roman force sent against him.
With the western empire suffering from an ever more desperate shortage of military manpower things only got worse.
Attila interpreted this as an offer of marriage and demanded no less than half the western empire as his dowry. Naturally this was rejected, which caused Attila to lead his terrifying Huns into Gaul.
The Huns captured and sacked Mediolanum (Milan), destroyed Aquileia and then marched on Rome. Aetius, without the Visigoths, could not possibly take on an army the size of Attila's and was forced to stand by as this happened.
Attila withdrew and in AD 453 died. His empire fell apart almost at once, the Hun menace to the empire disappearing as fast at it had arisen.
In AD 453 Aetius son was betrothed to the Valentinian III's daughter Placidia. Aetius thereafter was granted his fourth consulship in AD 454. He stood at the height of his power, having effectively ruled the western empire at the head of the military for two decades.
But with the powerful Aetius no longer in control, the court was gripped by intrigues.
On 16 March AD 455, Optila and Thraustila were among the guards of the imperial entourage as the emperor practised archery on the Campus Martius at Rome. Everyone was taken by surprise as Thraustila stepped forward and murdered Heraclius and Optila struck the head of Valentinian III, causing wounds from which he died.