The Arian Scourge: The Vandals!
The Vandal tribes came to the Roman Empire from Germanic territory in Poland. They were part of a confederation of tribes that included both German and Slavic tribes. Before other Germanic people were pushed out by the Huns, the Vandals were pushed out by other Germans. The Goths attacked the Vandals and pushed them westward into the Roman Empire.
The Vandals fought a series of wars against the Roman Empire as allies to the Suebi and Alans during the 2nd and 3rd centuries that resulted in them being settled along the Roman border in Pannonia, which roughly corresponds to modern Austria and Croatia. The Vandals, along with their allies the Alans and Suebi, accepted Christianity and moved across the Roman Empire towards Gaul.
In Gaul and Hispania
Upon arriving on the borders of Gaul the Vandals found that Gaul was already inhabited by Germanic tribes serving as foederati for the Romans. The Frankish Confederation had occupied the Rhineland since the time of Julius Caesar, and they had settled on the Roman side of the Rhine as military vassals known as foederati.
The Franks barred the Vandals from entering Gaul, but the Vandal armies overwhelmed them. The Vandals themselves were excellent foot soldiers, and their allies the Alans were some of the best cavalry in the world at that time. The Franks were defeated, but only pushed out of the Vandals way. The Vandals ran amok through southern Gaul sacking Roman settlements as they went.
When the Vandals arrived in Hispania they made a treaty with Rome and become Foederati in the north and south of the Iberian peninsula. The Alans settled in central Hispania, and the Suebi settled in the north between the Alans and Vandals. These three groups were foederati, meaning they were semi-independent kingdoms that had their own laws and they had Roman settlements inside their kingdoms that followed Roman law whom they defended.
When the Vandals accepted Christianity they accepted the teachings of the Bishop Arius. Arius taught that Christ was not God when he was on Earth. The Arians believe that Jesus Christ was wholly man when he was on Earth, and that he rejoined God when he ascended to Heaven. This was a highly disputed issue in the early Church, but at the Council of Nicaea the issue was put to rest by examining scripture.
Arius refused to recant his beliefs though. Most Roman citizens went Catholic, and the vast majority of Christians at the time went Catholic, but the Arians converted heavily among the pagan Germans. The Vandals chose Arianism and were considered heretics because of it.
The Vandals were openly intolerant of their Catholic subjects. Only Arians could hold government positions, and Catholic aristocrats and churches were heavily taxed. The Vandals Arian faith combined with their militarism made the Roman Empire find new allies.
The Visigoths were hired by the Roman Empire to settle in southern Gaul and Hispania to displace the Vandals. At the same time a Roman general in North Africa requested the help of the Vandals in securing the Imperial throne. The Vandals were simultaneously pushed out of Hispania and pulled to North Africa. The Alans and Suebi took the brunt of the Visigoth attack, their kings were killed and their kingdoms were destroyed. As a result the king of the Vandals, Geiseric, became king of the Alans as well.
When the Vandals crossed in to North Africa they had a large military force. They were supposed to come to the aid of the general Bonifacius who had fallen out of favor with Rome, but by the time the Vandals had arrived the general had reconciled with Rome. When Bonifacius dismissed the Vandals they decided to stay.
The Vandals invaded the Roman provinces, and seized Numidia, which was part of modern day Algeria and Tunisia. Peace was made with the Roman empire, but Geiseric soon breached the peace and seized Carthage, which became his capital. When he took Carthage Geiseric captured the Roman fleet anchored there.
The Vandals were brutal in North Africa. In Hispania they were somewhat tolerant of the Catholics, when they took North Africa they forced conversions and killed bishops wantonly. With the captured navy the Vandals took over the Balearic Islands, Sardinia, and Sicily. Italy itself was eventually invaded by the Vandals.
Geiseric made an agreement with Pope Leo I to sack Rome, but not kill it's people. In an attempt to create a lasting truce with the Roman empire Geiseric took the Empress Eudoxia and her daughters back to Carthage. The daughter of the Empress, Eudocia was to be married to Geiseric's son Huneric. Most Germanic tribes wanted some connection to the Imperial throne so they could try to claim the Roman Empire.
Geiseric died in 477, and his empire began to fall apart soon after. Huneric was not as capable as his father and his military forces weakened. When Huneric died the Ostrogoths took Italy and most of Sicily. The next king of the Vandals, Hilderic, granted Catholics the right to worship and he was killed by a rival faction that reinstated the anti-Catholic policies of most Vandal kings.
Justinian I, Emperor of the Eastern Roman Empire, wanted to reunite the Roman world, so he declared war on the Vandals with the intention of restoring Hilderic as a client king before the usurpers murdered Hilderic. Under General Belisarius the Roman army landed near Carthage and crushed the Vandal armies. Belisarius led the Roman armies across North Africa and reclaimed the entire coast from the Vandals.
Some Vandals fled the coastal areas and joined Berber tribes, while others went to Byzantium to serve as mercenaries. The Vandals ceased to be an independent ethnic group after the fall of their kingdom.