Later, Emperors such as Hadrian and Trajan sent campaigns into Germania (what is now eastern Germany, Austria, and Bohemia). The reason — to press the Germanic tribes back away from Imperial borders, & keep them subdued. Initially they were successful.
The Germanic tribes were under pressure from Altaic (Turkic or Hunnish) invaders to the east, however, & didn't remain at bay for long. The Romans, aware of the pressure they were under, tried to make accomodations. They offered to allow some of the Germanic tribes to move about the fringes of the Empire, & sometimes lent legions to help defend them during these movements. At first, they kept them outside of the Empire's own borders, but eventually, could no longer keep that up.
In the middle of the 4th century, Eastern Roman Emperor Valens allowed the Visigoths, a Germanic tribe, to occupy Roman territory. Specifically, he gave them the district of Thrace, which lay to the south of the Danube River & north of Greece. He did this with the condition that the Visigoths turn in all their weapons, & hand over their male children as hostages. They provided hostages but failed to completely disarm — out of fear that other tribes might come after them.
At about the same time, the Ostrogoths, another Germanic tribe, were overrun by Altaic invaders, & some of them fled as refugees, into Roman territory, just to the west of the Visigoths. There these refugees fell in with their Visigothic cousins, & hostilities with the Romans began.
Emperor Valens grew impatient, & believing he had the upper hand, decided to move against the Goths. He attacked while the Gothic cavalry was away on scouting maneuvers, hoping to win the day. As the battle progressed in his favor, though, the Gothic cavalry suddenly returned, sooner than expected, & attacked the Roman legions' flanks. The Romans were routed. Valens himself was slain in the battle.
Valens's successor, Theodosius I, established an uneasy peace with the Goths, but since the latter had a taste of victory, they wouldn't be so easily appeased. Additionally, the Altaic tribes continued attacking the Germanic tribes from the east, forcing the Germanic tribes deeper into Roman territory.
In Iberia, the Visigoths established their own kingdom, one which would last for a long time. The Vandals, who had meandered into Iberia via a long route (beginning in modern Denmark, they traveled down into Moravia, then followed the Danube westward, crossing the Rhine, then the Pyrenees Mountains), ran into them. The Vandal king, Gaiseric, led his people across the Strait of Gibraltar, into northern Africa. Rome had few defenses, here, so the Vandals swept across Libya in no time. They were halted only by the Roman legions of Egypt, & were fenced in to the western half of the north African coast.
In the meantime, further to the north, the "empire" of Attila the Hun — actually a coalition of his own Altaic/Turkic nomadic tribe along with some others, including a contingent of Ostrogoths & Slavs — had overrun most of Germania, and had entered Roman territory in the Rhine region, heading west. The Roman general Aetius & his legions, with the Visigoths as allies, defeated Attila at Chalons in Gaul, in 451, & within 2 years, Attila was gone, along with his "empire." Freed of control by Altaic nomads, the Ostrogoths migrated into Italy.
The Ostrogoths initially didn't enter Italy as invaders, but as emigrees & refugees. They quickly adopted Roman culture & many of them joined in the ranks of the Roman legions. They became a buffer of sorts, protecting central Italy from incursions by other Germanic tribes. Their king, Theodoric the Great, became sole ruler of Italy, with the death of the last Roman Emperor in the West, in 502.
By this time the Roman Empire in the west, had ceased to exist. Northern Africa was a Vandal kingdom; Iberia was a Visigothic kingdom; northern & western Gaul was a Frankish kingdom; southeastern Gaul was a Burgundian kingdom; & Italy was an Ostrogothic kingdom.
In addition to being ruthless & a capable leader, Justinian had an equally capable general, named Belisarius. Together, they plotted a campaign to rebuild the old Roman Empire, & reclaim the west, which had been lost to the various Germanic tribes.
After many years of hard fighting, in many theaters of battle (particularly in Italy & in northern Africa), Justinian and Belisarius came very close to accomplishing their goal. They broke both the Vandal & Ostrogothic kingdoms. Southern & eastern Italy belonged to them, along with several portions of northern Africa near cities they'd been able to reclaim (such as Hippo & Carthage).
But able leaders usually have their flaws, and this was as true of Justinian as anyone else. He had many enemies at home in Constantinople & Greece, his power-base. Furthermore, his wars abroad were costly. At several points, he & Belisarius had to interrupt their campaign to return home, either to put down internal strife, or because funds for their war had run low.
Although these campaigns had shown a great deal of promise — & were more successful than Justinian's detractors had predicted they'd be — his reign ended, without the restoration he'd hoped for.
The Romans' defeat at Adrianople greatly damaged this reputation. The legions weren't nearly so intimidating, any more. They had weaknesses, which could be exploited. Subsequent invaders feared them much less than they had before. Barbaric kings such as Alaric of the Vandals dared to think they could invade Roman territory, & even attack (& sack) imperial Rome, itself!
The Battle of Adrianople was, indeed, a point of no return. Rome never recovered from this defeat, and the Germanic tribes never looked back. Even the able & capable Justinian couldn't undo the damage that had been done.
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