As the Vandals descended upon north Africa, the Huns reappeared as Eastern and Western Rome's primary nemesis. Geographic stasis in the Balkans just beyond the Danube from the 380s and subjugation of resident Gothic and Suevi remnants had caused a political change in Hunnic society. Though previously the Hunnic hordes recognized no unitary political leadership, by the 420s Rouia (Rugilla) emerged as their overlord. He began to steeply increase the tribute Constantinople had to pay as ransom for those lands precious to the Eastern Emperor. In 433 this tribute was doubled again to 700 yearly pounds of gold, and the Hun leader, Attila, additionally demanded that Emperor Theodosius II return all Germans who had fled Attila's wrath. Extortion continued to 441, when Attila took a mixed Hun-German army over the Danube. Ravaging valuable agricultural lands, he withdrew only after increasing tribute to 21,000 pounds of gold. Six years later, he returned, pillaging Balkan and Thracian cities, and demanding imperial evacuation from land south of the Danube equal to five days' march. In 450, the new Eastern Emperor Marcian (450-457) refused a further tribute increase.
Marcian was saved from destruction by the Western Emperor Valentinian III's daughter, Honoria. Rejecting her father's command to marry an aged senator, she requested Attila's protection. Reading this as a marriage proposal, in 451 he came West for his bride, demanding a dowry of the western half of the Empire. Aetius, who had heretofore relied on Huns to rein in Germans, was forced to change course and turn to Germanic tribes for soldiers. Equally terrified of the Huns as were the Romans, the Franks, Burgundians, Alans, and Visigoths supplied him with troops. In 451, at the Battle of Catalaunian Plains, Aetius and Theodoric (who died during the battle) defeated Attila. Visigoths played the main part, while Roman regulars were nearly absent. Attila returned the next year, crossing totally undefended eastern Alpine passes into the Po Valley and Northern Italy. Aetius was unable to recruit Germans, as the region was of no concern to them. Plundering a prosperous region, Attila withdrew without proceeding to Rome. It may be, as some versions hold, that a party of Senators and Pope Leo I convinced him to relent. Alternatively, a plague among his troops, or recognition that the terrain was inappropriate to his horse-borne forces, could have convinced him to leave. In 453, he took a new barbarian bride, dying the night of his wedding. In this political vacuum, subject Germans revolted against the Huns under Gepid leadership. The Germans won, and the Huns scattered, no longer impinging on European history.
Feeling he no longer needed Aetius and resenting his closeness to the Huns, Valentinian III had his Master of Soldiers killed. Valentinian himself was murdered in 455 by Barbarians of Aetius' retinue. That same year, the North African Vandal leader Gaiseric sent a pirate fleet up the Tiber River, sacking Rome and plundering it heavily for fourteen days. The next twenty-one years were the practical end of the Roman state, and saw a series Germanic generals who controlled puppet Western Emperors, and through those Emperors cared only for Italy and, at times, North Africa. One Ricimer, of Visigothic and Suevian origin, defeated the Vandals at sea in 456. In 460 he and his emperor Marjorian set out to regain Africa, but the imperial fleet was decimated. In 461, another puppet Emperor of Ricimer's (he had discarded Marjorian) was not recognized by Gaulic Roman forces, so areas north of the Loire slipped from Italian control. Ricimer began to favor rapprochement with the Vandals rather than war, and succeeded in this endeavour by installing as Emperor an unimportant senator with a slight familial relation to Gaiseric by marriage. Ricimer and the Emperor had died by 472, at which time areas under imperial control had further contracted to include only Italy and Southeastern France. South of the Loire in central and Western France was Visigothic, while South-Central France was Burgundian. Ricimer's successor as Master of Soldiers, Orestes made his own son Emperor Romulus Augustulus in 475. The two were both overthrown in 476 by the Barbarian-Roman general Odovacar. Not setting up a puppet emperor as had become fashion, Odovacar, supported by senators, notified the Eastern Emperor Zeno that there was no need to appoint a Western colleague: Odovacar would rule the West as Zeno's agent, and thus was sealed the official end of the Western Roman state. Zeno seemed to acquiesce, then sent Theodoric the Ostrogoth to unseat Odovacar in 488-93 as a way to prevent the Ostrogoths from causing more damage in the Balkans and Thrace. Theodoric succeeded, and became 'king of Italy'. As had countless Barbarians before him, Theodoric presented himself as a Roman official. Along with the Burgundian and Visigothic kingdoms, his realms gave form to the first post-Roman, Medieval order, soon to be joined by the Franks from the 520s.
We are left asking how such an ignoble end could encompass the once glorious Western Empire. Countless reasons have been offered. In terms of immediate causality, it appears that policy incoherence and a lack of resolve by emperors and other elites combined to sap all resilience out of the Roman government. What was missing was the will-power to break the cycle of Germanic military strongmen in Roman ranks, the senseless intriguing of Roman politics, and the general relinquishment of government responsibilities. Indeed, for its last eighty years Rome appears to have been bereft of the spirit of past rulers, such as Augustus, Vespasian, Diocletian, or Constantine. Was it simply the magnitude of military and fiscal challenges that overwhelmed uninspired leaders? This is possible, though it should be recalled that the sedentary population of the Empire, while decreasing in the fourth and fifth centuries, was still much larger than the population of invading Germanic tribes. Reviewing the sequences of Germanic infiltration into Roman military, administration, and society, it seems that rather than falling, the Roman state in the West willingly gave up, letting day to day control of its holdings slip from its fingers without so much as a spasm, delegating itself out of existence. It is not even clear that those responsible for this irreversible delegation were even aware they were presiding over the destruction of a state. Did they see their world as a continuation of Roman policies and methods dating back centuries, with potential for preserving the state?
Moving to theories of the long-term causes, some are quite far-fetched. Debilitating malaria epidemics among the ranks have been posited, just as some have suggested that the use of lead piping in aqueducts, sewerage, etc. in Roman cities caused a gradual lead poisoning and inability to conceptualize complex solutions. Such ideas are unquantifiable. More serious is the notion that the city-state was the basis of civilization in Antiquity. With its economic and then demographic decline from the mid- second century, the intellectual and pragmatic problem-solving vitality of the Empire diminished. This is not unrelated to the theory that from the ascendance Severi, as more of the empire's rulers were raised in Balkan areas or regions far from long-time Roman cultural centers, they were unable to conceptualize 'Rome' as a civilization, and unable to distinguish it from lesser cultural forms. As Rome began to fail, then, few noticed, as they could no longer recognize what Rome actually was. This thesis also applies to those proleterians or former slaves that were able to rise through the ranks of army and bureaucracy based on Diocletian and Constantine's reforms. These men, the power in Rome, were protecting a world and ideal they did not fully understand, and their protection was therefore haphazard and incomplete.
On a much more abstract level are two suggested reasons for the fall. According to one, after 200 CE, the Empire became 'Orientalized' in the ethnic complexion of its rulers and administrators. This does not seem to count for much in real policy; indeed, the Eastern Empire was much more 'Oriental', and it outlasted the West by half a century. Still, some see Christianity and other mystery religions as a philosophy and world view imported from the East, and perceive of these religions as both sapping Roman rational thinking and removing from Roman imperial service talented people who chose church service. Only the latter aspect appears to have merit, and that only in the most guarded sense.