Flavius Aetius was a statesman and general who lived and fought during the mid 5th century CE, when the Roman Empire in the west finally hit bottom after its long death-spiral. He was politically powerful and militarily successful, and has often been wistfully eulogized as "the last of the Romans." A careful review of the evidence tends to indicate that this apellation is correct - he was the last defender of the Empire because by the time he was done there wasn't much left to defend.
Aetius was born around the turn of the 5th century, the son of a successful magister equitum (cavalry commander) who fought for Constantius in Gaul. His father's prominence led to Aetius' being sent as a hostage first to the Gauls, and then to the Huns. During this period he made a number of strong contacts with the Huns, and first met a youth named Attila, who was himself soon sent to Rome as a hostage. The two men would meet again.
It is not certain when Aetius returned to the Empire, but he soon made his debut on the stage of imperial politics - and warfare. When the emperor Honorius died with no heir in 423 CE, his designated heir - Valentinian III - was far away in the eastern capital of Constantinople. The western army had no interest in Valentinian, and elevated a civil servant named John to the position of emperor instead. The infant Valentinian (or rather his mother Aelia Galla Placidia, who wore the pants in the family) set forth with an army to assert his claim to the throne, backed by the eastern emperor Theodosius II. Aetius was dispatched to recruit an army of Hun mercenaries to hold Valentinian's armies at bay. He returned at the head of a sizeable force (which quite possibly included the young Attila) only to find that John had been defeated at Ravenna by Placidia and executed in the spring of 425.
Thus, Aetius and Placidia found themselves in uncomfortable positions - he leading an army in the service of a dead man, she facing down an able general - leading a fearsome horde of Huns - whose emperor she had just ordered killed. A deal was brokered. The Huns returned home (no doubt with much fuller pockets,) and Aetius went to Gaul to become magister militum of the army there. He pursued several successful campaigns against the Visigoths (who had been invited into the Empire almost a century before, to help defend it) and in 429 he was granted the post of junior magister militum in Rome. A year later, he arranged the death of his superior - the ironically-named Felix - and became the preeminent general of the Western Empire.
Placidia, probably alarmed by and jealous of Aetius' rapid rise to power, brought Boniface - the general of the Roman provinces in Africa - to Rome in 432 and gave Aetius' title to him. Boniface had long been a rival of Aetius, and was eager for a chance at revenge, but Aetius wasn't one to give up easily. A brief civil war ensued, in which Boniface's armies triumphed but the general himself was slain. His son-in-law Sebastian took his place, but couldn't resist, and within a year Aetius had secured political and military supremacy over the entire West. Around this same time, Attila and his brother Bleda took control of the Huns.
Aetius immediately returned his attention to the ever-more-aggressive Visigothic kingdom, having made the decision that Gaul was - outside of Italy - the only imperial province that deserved special attention. He hurled his Hun mercenaries against Theodoric I, king of the Visigoths, in an attempt to maintain (or rather regain) direct imperial rule in the area. In the meantime, pleas for assistance from Britain for aid against the Picts and Scots and from Galicia, which was being overrun by the Sueves, were ignored. More seriously, the Vandals were able to seize North Africa - the "granary of the Western Empire" - unopposed, and by 442 the entirety of the Roman dominion there belonged to them.
Aetius' strategy of relying on the Huns for manpower depended on one thing - the Huns; and once Attila had disposed of his brother in the early 440s, the Huns belonged to him. The emergence of a single authority made the Huns a powerful force, and they were able to extort a great deal of money from Theodosius in the east. Theodosius died in 450, however, and his successor refused to pay any further bribes. Attila, for reasons not fully explained (although a mistaken offer of marriage may have been involved) marched his fearsome horde into Gaul.
For Aetius, this was a Very Bad Thing. The cornerstone of his military power was marching against him, and ironically, the only recourse he had was an alliance with Theodoric and the Visigoths. Ally he did, and a mixed (although predominantly Visigothic) army met Attila on the Catalaunian plains, near the modern-day town of Chalons-sur-Marne. Aetius and his uncomfortable allies prevailed, and Attila withdrew, leaving Theodoric slain on the battlefield. It is not certain whether Aetius prevented a successful pursuit of the Huns because he wished to continue his campaign against the Visigoths or whether he was simply unable to continue the fight, but the Huns withdrew unscathed. Attila would not be denied for long, however.
In 452, the Huns returned. They sacked Mediolanum (Milan), destroyed Aquilea, and marched on Rome. They were confronted by two senators and by Pope Leo I - and, inexplicably, turned back. The typical Christian version of the story is that Attila was humbled by Leo's holiness, but an epidemic amongst the Hun forces is more likely. Whatever the reason, Attila turned away from Rome and returned north, only to die a year later. The power struggles following his death, accompanied by several major military defeats, caused the powerful Hun Empire to disappear like smoke on the wind, never to be seen again.
As Aetius' career had waxed with his Hun counterpart, so did it collapse after his death. The source of his military strength had turned against him, he had failed to defend Italy, and with the disappearance of the Huns as a coherent military force his future prospects for success were grim. Worse, he still had powerful enemies. Placidia, Valentinian, a former praetorian prefect named Petronius Maximus, and the Emperor's chamberlain Heraclius plotted his death, and in September of 454 Flavius Aetius, "last of the Romans," died at the hands of Heraclius and the Emperor himself.
Ironically, Aetius' death spelled the end for the conspirators as well. Petronius Maximus had expected to be promoted to the post once held by Aetius, but was denied. He conspired with Aetius' former personal guard - Scythian warriors, sworn to avenge their fallen lord - and within six months of Aetius' death, Heraclius and Valentinian were cut down. Placidia, having no legal power of her own to wield, fell into obscurity, while Petronius managed to get himself declared emperor. The ever-opportunistic Vandals, however, smelled weakness and set forth to give Rome a thorough sacking. The wretched Petronius Maximus was killed by a brick hurled by an enraged citizen as he ignominiously fled the city.
Within 30 years the socio-political machinery of Roman government finally ground to a halt. Aetius had given away North Africa and the rest of the Empire, and he had rendered the office of emperor irrelevant. The dynasty that had ruled the Western Empire since 364 died out, and the succession of military dictators that followed concentrated on regaining Africa, only to lose Gaul. The political cohesiveness of the Empire was irrevocably shattered, and although many of its social institutions lived on, the greatest empire the world had ever known was no more.
Abels, Richard, "Roman Military and its Grand Strategy." http://www.nadn.navy.mil/Users/history/abels/hh381/Romimparmy.htm#LATE%20ROMAN%20MILITARY%20(from%20Diocletian%20to
Collins, Roger, Early Medieval Europe: 300-1000. St. Martin's Press, NY, 1991.
Lupo, Rosa, "Attila and the Huns: Horsemen of the Apocalypse." http://www.glilli.com/at.htm
"'Valentinian III: Flavius Placidus Valentinianus (AD 419 - AD 455)" From "The Illustrated History of the Roman Empire," www.roman-empire.net.