"How big a fiscal hole the loss of North Africa made in the western Empire’s budget is impossible to say, but we can work out the reduction in the armed forces implied by the revenue lost from just Numidia and Mauretania Sitifensis…. [T]otal tax lost from these provinces, because of the new remissions, amounted to 106,200 solidi per annum. A regular comitatensian infantryman cost approximately six solidi per annum, and a cavalry trooper 10. This means that the reduced tax from Numidia and Mauretania alone implied a reduction in army size of about 18,000 infantrymen, or about 10,000 cavalry. This, of course, takes no account of the complete loss of revenue from the much richer provinces of Proconsularis and Byzacena, so that the total of lost revenues from all of North Africa must have implied a decline in military numbers of getting on for 40,000 infantry, or in excess of 20,000 cavalry….
"We don’t have an updated version of the Notitia Dignitatum’s army lists (the distributio numerorum) for the early 440s, but if we did, they would certainly show a further substantial deterioration since 420. Only a massive new threat, therefore, could have made Aetius call off the joint east-west expedition and accept these disastrous consequences. Where had this threat come from? Merobaudes, in the surviving fragments of the panegyric of 443 at least, is allusive rather than explicit. Bellona, goddess of war, comments: ‘I will call forth nations situated far away in the North, and the Phasian stranger will swim in the fearful Tiber. I will jumble peoples together, I will break the treaties of kingdoms, and the noble court will be thrown into confusion by my tempests.’ Then she issues her orders to Enyo: ‘Force savage crowds into war, and let the Tana’is, raging in its unknown regions, bring forth Scythian quivers.’ Arrow-firing hordes from Scythia? In the middle of the fifth century, that could mean only one thing: Huns.
"And the Huns were, indeed, the new problem, the reason why the North African expedition never set sail from Sicily. Just as it was making final preparations to depart, the Huns launched an attack over the River Danube into the territory of the east Roman Balkans. Constantinople’s contingent for Carthage, all taken from the Danube front, had to be recalled immediately, pulling the plug on any attempt to destroy Geiseric. Yet all through the 420s and 430s, as we have seen, the Huns had been a key ally, keeping Aetius in power and enabling him to crush the Burgundians and curb the Visigoths. Behind this change in attitude lay another central character in the story of Rome’s destruction. It’s time to meet Attila the Hun…"
--Peter Heather: The Fall of the Roman Empire : A New History of Rome and the Barbarians