The invasion of North Africa was not such a crazy endeavour as it may seem. Taking into consideration the whole context, it made total sense for the Vandals to trade Iberia for the Roman African province. 

Ripe for the taking

First of all, the Roman governor of the African province was at that time in open rebellion against the court of Valentinian III. In the chronicles of the time he goes by the name and title of Count Boniface, and was not unknown of Genseric himself. It is said that in 428, he was ordered to invade Iberia by the south while a Roman army was invading from the north, but that disobeyed or ignored the orders which could have ended the Vandals there and then.
Hence the suspicion of the court at Ravenna and the rumours that he was actually in league with Genseric. Some suggests that he saw a great opportunity to boost his own power by inviting the Vandals in North Africa, to settle them there and therefore shore up his defenses in exchange for half of the territory. If it was true, he completely misjudged the cunning Genseric and his plan backfired spectacularly.  But there is no real evidence of that.

                    A mosaic found in Carthage showing a wealthy estate in North Africa [Source]

Second, the Roman province of Africa was not so much “loyal” as “submitted.” A lot of Berbers tribes - untamed by the Roman way of life - were still roving at the fringe of the desert, always ready to pounce on cities and big estates at the first sign of a weak defense. And among the Berbers which actually saw themselves as Romans, and converted to Christianism, an heretic sect had taken root, called Donatism in the 4th century. They had been mercilessly persecuted by the Romans Catholics but a lot of them were surviving. For this part of the population, the arrival of the Vandals (who were Arians, another Christian sect) was seen as a liberation or at least as the end of oppression.

And last point, Genseric knew that in Iberia the Vandals were completely exposed to a counter-attack from the Romans. Even if they had no immediate enemy around, they didn’t have enough manpower to effectively secure the Pyrenean passes and all the ports of the Mediterranean coast. And we know that because, before to board ships and cross the Straits of Gades (modern day Straits of Gibraltar), Genseric ordered a general census of the Vandal people. In total, Genseric had under his orders 80.000 people, which meant something between 15.000 to 20.000 warriors. Even if battle-hardened, it was impossible to control the whole of the Spanish peninsula with that number, but more than enough to smash into North Africa - which did not have any field army capable of rivalling that number.

                                               A collector card showing the landing of the Vandals [Source]

The regrets of Boniface

The Vandals crossed the Straits of Gades in 430. If we follow the theory that Boniface invited them over, that may explain the ease with which they reach the shores of Africa and how fast they spread in Mauretania and Numidia. However, it may also simply be the cause of their winning over the friendship of the local population. However, that does not explain why, by the end of 430, they are knocking at the door of the town of Hippo - 1500 kilometers in the west, on the doorstep of Carthage. 

In his “History of the War of the Vandals”, the historian Procopius explains that:

[Boniface] repented of his act and of his agreement with the barbarians, and he besought them incessantly, promising them everything, to remove from Africa. But since they did not receive his words with favour, but considered that they were being insulted, he was compelled to fight with them, and being defeated in the battle, he retired to Hippo Regius, a strong city in the portion of Numidia that is on the sea. There the Vandals made camp under the leadership of Genseric and began a siege.

The Vandals were unable to conquer the city and Boniface reconciled himself with Rome and got reinforcement from Byzantium, led by a general of Alan origins, called Aspar. Now more confident, Boniface confronted the Vandals in a pitch battle… which he lost, and had no other choice than to lock himself up in Carthage, while Genseric retook the siege of Hippo, which fell in July 431. Genseric had now in his power both Mauretania and Numidia. Would he press his advantage and conquer Carthage right there and then?

                                                                                            [Source]

Rome vs Carthage 2.0

          

                                                The port of Carthage at the height of its power [Source]

Procope tells us that: 

At that time Genseric, after conquering Aspar and Boniface in battle, displayed a foresight worth recounting, whereby he made his good fortune most thoroughly secure. For fearing lest, if once again an army should come against him from both Rome and Byzantium, the Vandals might not be able to use the same strength and enjoy the same fortune [...] and so he made a treaty with the Emperor Valentinian providing that each year he should pay to the emperor tribute from Libya, and he delivered over one of his sons, Honoric, as a hostage to make this agreement binding.

This treaty comes at the right time for both parties. Rome is still confronted by the Hunnic menace of Attila and needs the grain from North Africa to feed its people, while Genseric needs time to consolidate his hold on North Africa. So, this is nothing more than a truce: Rome does not recognize the Vandals as equal partners, nor Genseric as on the same level as Valentinian III. Each side catches up its breath before the next game, but once again Genseric is faster than anyone else and four years later the Vandals enter by surprise the town of Carthage (439).

Far from resting on his laurel, Genseric spends the next years building a state from the ground up, rooting out support for the Catholic church, establishing an aristocracy and a bureaucracy to sustain his hold on power, breaking up the big estates of the Roman nobility and offering them to his people.

And the enterprising Barbarian didn’t stop there. Probably motivated by the tales of the Phoenician navigators who founded Carthage in the first place, he decided to remind his people about their long-forgotten origins, when they were settled along the Baltic sea - living from fishing and the odd piracy… Well aided by all the boats and ships available in the port, the Vandals reinvent themselves as pirates and starting infesting the Western Mediterranean, raiding the coasts, seizing every ships they find in their way.

They made of themselves such a nuisance that the Eastern Emperor Theodosius II himself decided to step in, in lieu of the useless Western Emperor. He assembled a 1100 ship strong fleet in order to crush and oust once and for all the buggering Vandals. Seeing the writing on the wall, Genseric immediately changed tack and countered with diplomatic overtures. Well helped by the fact that the Huns chose this precise moment to invade the Balkans, Genseric probably let a huge sigh of relief when the invasion fleet stopped right at Sicily before to go back. .

In 442, Valentinian III eventually capitulates: in recognition of the Vandal Kingdom, Genseric has to give back Sicily and Mauretania (nowadays Algeria and Morocco) but he has what he wanted: a legitimate kingdom for his people.

After years of wanderings, the Vandals have now a firm hold on their new territory. Will they eventually kick back and trade the sword for the plough and bask in the African sunshine? You obviously don’t know your Genseric...
 

To be continued...

Previous episodes

Enemies of Rome 4.1 - GENSERIC - “The Vandals Assemble”

 Sources

http://www.ancient.eu/Vandals/

http://www.roman-empire.net/articles/article-016.html  

http://remacle.org/bloodwolf/historiens/procope/vandales.htm