Belisarius was not in command of any legions during the Vandalic and Gothic Wars. The legions of the Roman Empire had largely been replaced by new units by the reign of Justinian. I know some legions still survived, for example the Legio V Macedonia that was founded by Augustus in 43 BC was in Egypt in the 7th century when it was conquered by the Arabs. That being said, I’m going to discuss the troops that Belisarius was in command of, and their size.

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To start, Belisarius actually had around 15,000-17,000 men when he invaded North Africa. This army was composed of roughly 10,000 infantry and 5,000-7,000 cavalry (of which approximately 1,500 was his personal Bucellari). I don’t know where you read that his army was mostly composed of Arabic and Slavic mercenaries, but this is not true. The Slavs had only just appeared in the historical record by the time of the Vandalic and Gothic wars, and while they did fight in the wars of reconquest they are not mentioned by Procopius until the first siege of Rome, and even then there was not very many of them. The Romans had of course had some level of contact with the Arabian Peninsula for centuries by this point, but I don’t believe they had ever had a sizable presence in the military (at least not up to this period). It is true, however, that a large portion of his force was made up of non-Roman “mercenaries”. Mercenary isn’t the most accurate term to use here as most of these men would have been treaty bound allies, but it’s also not inaccurate so we’ll leave it be. These men were mostly Huns and Heruli. There were approximately 600 Huns and 400 Heruli all of whom were mounted troops. The infantry was mostly recruited from the Comitatenses of the field army, but there were some foederati in their rank as well. I’d also like to point out that a large number of the comitatenses troops would have been Isaurians, who were Roman subjects but oftentimes they were viewed as barbarians and described as such.

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This force was actually quite small for a Roman army, even at this point in time, and even the Vandals could field a larger force than what Belisarius had at hand. In addition the Vandals already had a formidable reputation when it came to warfare, albeit most of that stemmed from the reign of Genseric 60 years before this. But it should be kept in mind that Justinian had literally just signed a peace treaty with Persia in order to invade North Africa and would have needed to keep a substantial number of troops along the border. On top of this the lengthy Danube border needed to be defended as well, so Belisarius had to make do with what he had. Luckily for Justinian, Belisarius was a talented and extremely capable general.

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That being said, he was still outnumbered by the Vandals, who might have fielded an army as large as 40,000 men (estimates range from 20,000-25,000 or 30,000-40,000). Even with the numerical advantage, massive though it might have been, the Vandals were undeniably outclassed by the Romans. Belisarius’ army was a professional one and many of his men had probably fought with him against the Persians. The Vandals did have some recent conflicts with the Moors, but nothing on the scale of Justinian’s war with Persia, and their army was not a professional one. In addition, the Vandal’s army was almost entirely cavalry, which is not a problem in and of itself (the Battle of Carrhae is a testament to this), but they were mostly light cavalry who used javelins and bows to harass their enemies. The Roman cavalry, on the other hand, was mostly Cataphracts and the horse archers supplied by the Huns and Heruli. The Cataphracts were heavily armored and used both melee weapons and bows. This meant that the Romans could fire their bows into the Vandals, who would have been less protected due to their lighter armor, and then charge in with their lances. The Vandals stood little to no chance in close quarters combat with the Romans, whether it was the cataphracts or the heavy infantry.

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Belisarius was able to defeat the Vandals in less than a year, but they proved to be a difficult people to pacify. This, and the ever present threat of Moorish raids, meant that Belisarius had to leave a large portion of his army behind in North Africa to maintain control. This meant that he had even fewer men available to him when he invaded Italy. In fact the army he had at the beginning of the Gothic War was only half the strength of the one he used to invade North Africa at roughly 7.000 men.

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While this army was certainly much smaller and would be facing a much larger, and better, army than the Romans had fought in North Africa, its small size likely had a strategic purpose on well. But it should also be noted that Justinian sent an army to capture Salona in Dalmatia that could then be used to reinforce Belisarius if needed. Now back to the size of this force, these 7,000 men were mostly infantry, possibly because most of the cavalry was still tied up in North Africa or along the Persian border. The only cavalry he had available was his personal Bucellari and possibly some Huns and Heruli, so maybe a little over 1.500 total cavalry. Its possible that Justinian didn’t give Belisarius more men is because he was not yet fully committed to an all out war with the Ostrogoths, and thanks to Procopius we do know that he was still negotiating a peaceful exchange of Italy with the Ostrogothic king Theodahad. Belisarius’ initial orders lends credence to this idea, because he was ordered to capture Sicily through peaceful means if possible, and if not then he was to sail directly to Libya and act as if that had always been his original destination. The bulk of his infantry was also composed of Isaurians who were the elite infantry of the Roman empire at this time were. It should also be noted an army of 7,000 men could move faster than one of 15,000 men which would offer some advantages and allow Belisarius to quickly march through southern Italy. On top of this, its possible that Belisarius and Justinian did not expect much resistance from the native Italians, who still considered themselves to be Romans, and therefore might have thought that he could simply reinforce his army on the march as he made his way to Rome.

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Nevertheless, in June of 536 Belisarius was ordered to invade Sicily immediately and all pretenses of occupying the island peacefully were thrown out. Belisarius quickly faced opposition as Naples refused to surrender and the Romans had to besiege the city for a month. After this however the Romans did make quick progress towards Rome as most of the cities in southern Italy surrendered without a fight. The Romans were also helped by the fact that Theodahad was largely incompetent ruler and took almost zero preparations to resist them, including his refusal to send reinforcements to Naples. After this, however, Theodahad was overthrown by Vitiges, who was a Gothic general, and he proved to be a much more formidable leader. Nevertheless the Romans captured Rome with ease as the city surrendered without a fight, and Belisarius let its Gothic garrison leave unharmed.