In 375, westward-expanding Huns from Central Asia slammed into the Black Sea region Alans and Ostrogoths, routing them in battle with their cavalry-borne assaults. Those not enslaved fled frantically, displacing all in their path. In the next years, the limes erupted. Terrified by the experience of their northeastern neighbor, the Visigothic king Fritigern implored Valens to allow his tribe to migrate into Roman realms and settle south of the Danube. Valens delayed, then acquiesced in 376. Though the agreement was Visigothic disarmament in return for Roman provisions, the arrangement quickly broke down. Rising Roman mistreatment of Visigoths who had already suffered from forced migration, as well as non-arrival of food and some atrocities reflecting Greek distaste for their new neighbors, caused the Visigoths to revolt at the end of 376. By 378, Valens had arrived from Antioch with an army, yet failed to await Gratian's reinforcements and was routed at Adrianopole (modern Edirne) after Ostrogothic cavalry came to their brothers' aid. Valens was killed. Theodosius (379-95) controlled the damage by settling the Visigoths en masse as foederati on the Roman side of the border in Bulgaria—the first time Barbarians had been allowed into the empire as an entire group. After decimation of the Eastern Roman army, the Visigoths were welcome new soldiers. Theodosius began whole-scale Germanization of both Eastern and Western military forces at all levels. Resentment among remaining Roman officers and soldiers caused antagonism in the ranks. Theodosius also coaxed the Ostrogoths back over the border out of Roman lands, as the Huns had receded eastwards.
Theodosius was succeeded in 395 by ineffective emperors in both East and West, who did not entirely honor his agreements with the Visigoths. In that year, the activist Alaric was elected Visigothic king, and decided to migrate further into Europe and Italy in search of food and pasture land. Six years later, a mostly Vandal-Burgundian army led by Rhadagaesius crossed the Danube into the northern Alps. In both cases the impetus was a Hunnic return to Southeastern Europe, pushing all before them. Pandemonium ensued in Roman lands.
In 401, Stilicho, the Frankish-Roman military leader, went to subdue Rhadagaesius, allowing Alaric to bring his tribe into northern Italy and engage in limited pillaging. Stilicho defeated Rhadagaesius' forces, however, incorporating them into his army, which pushed Alaric back across the Italian Alps, where he decided to bide his time. Responding to continuing Hunnic pressure, and seeing the withdrawal of Stilicho's army to deal with the Visigothic incursion, Rhadagaesius returned in 405 and 406. Instead of plundering then withdrawing as in the past, he led a great Barbarian army dominated by Vandals, and including Alans, Suevis, and Burgundians, across the frozen Rhine at Mainz. Fanning out across Gaul, they attacked cities and agricultural areas, forcing the indigenous population into the hills. Neither sparse Roman forces nor Frankish auxiliaries could stop them, and eventually, Vandal led-forces crossed into Iberia, settling within Roman lands.
Alaric, seeing this and determined to secure better settlement for his tribe, was inclined by 408 to follow the Vandals into Gaul. Events intervened when the Western emperor Honorius ordered Stilicho's assassination. The Roman-German imperial army in Italy then split along ethnic lines, with Roman soldiers massacring the families of their erstwhile comrades. Then, the German elements defected en masse to Alaric's Visigoths. By 408, there was no army in Italy, so Alaric was able to enter the region and remain for four years. At first, a barbarian sacking of Rome was staved off by bribing Alaric; he returned in 409, however, wanting to negotiate a real settlement involving reintegration into the Roman army and provisions, but Honorius refused to negotiate and fled for Ravenna. A senate-elected emperor, Attalus, consented to Alaric's demands, but could not provide sufficient foodstuffs because North Africa, the capital's bread-basket, did not recognize him. Thus, in 410, the Visigoths undertook the first sack of Rome, though it was limited in duration and severity. The Goths then moved south to obtain a fleet for transport to North Africa and its food. The fleet was destroyed by a storm, and Alaric died. The Goths returned north with Athaulf as new king.
Requiring reconsideration is the significance of the 378 Visigothic defeat of Roman forces at Adrianopole. Theodosius was able to quickly resolve the situation, and though the Eastern Army was decimated and the Eastern half of the Empire was in most direct danger, it was able to continue for more than 1000 years more. Still, the great importance of the defeat was three-fold: 1) Theodosius' agreement to settle Alaric's people within Roman borders was new, and set a precedent, since it was repeated for all others who appeared to acquiesce to Roman suzerainty in the Visigothic manner. 2) While use of Barbarian slaves in Roman forces and employment of Barbarian auxiliaries directly on the other side of Roman borders had already occurred, from the 370s, the whole sale Germanification of Roman legions began. Though perhaps not understanding entirely what Rome was, Barbarian soldiers were no less loyal than their Latin counterparts (whose loyalties were at times in question); the Germans were perhaps even more versatile and willing combatants. Furthermore, Barbarian military employment was not limited to field forces. Stilicho, the Master of Soldiers until Honorius assassinated him in 408, was himself a Vandal, though it is likely that he saw himself as being as Roman as his predecessors. Ironically, then, Germanics like Stilicho and all who followed him as Master of Soldiers comprised all that was left of Roman military might to limit Germanic encroachment, or channel it away from Imperial interests. 3) After the death of Theodosius in 395, under whom the Empire was briefly united in name under one Emperor, Eastern and Western emperors would have increasingly less interaction. In some cases this mutual turning away was related to the use of Barbarian soldiers; when it was popular in one region, the court in the other area would turn against it. Beyond that, though, the capabilities of the two halves were so constrained by immediate local challenges that the luxury of intervention no longer existed. Of course, that the Visigoths (and later the Huns) elected to go west likely saved emerging Byzantium, whose only policy recourse during the dark years from 380-430 was to attempt to mediate and thus restrict Barbarian access to Mediterranean coastal areas, ocean vessels, and grain supplies.
To the extent that Roman policy failed during this period, due consideration must be given to the role of poor imperial policy and individual emperors' stupidity. Honorius' constant bickering with his Eastern colleague was not helpful, just as the latter's support of counter-claimants in the West could only be debilitating and wasteful of military manpower. Also, Honorius' flight to fortified Ravenna without fending off by force or negotiation the Visigothic sack of Rome has been interpreted as lack of will, cowardice, or stupidity. Interpreting his actions differently by assuming he actually had a policy, it could be said that he had a difference of opinion with Stilicho as well as with the Senate. Stilicho preferred to fight, while the Honorius was ready under duress to respond favorably to Alaric and Athaulf's demands. According to this view, Honorius recognized that force would not carry the day, that acquiescence to Visigothic demands would open the door to their settlement in Italy (as opposed to elsewhere in Roman lands), and that Rome was no longer strategically significant. Perhaps, then, the Visigothic departure from Italy in 412 could be read as Honorius' eventually Pyhrric victory.