I haven't found anything about their armies, only that they were great traders.
There were several Christian enclaves south of Egypt beginning about the time Christianity came to Alexandria. A lot of people don't realize the role Ethiopia (Nubia) played in spreading Christianity. Take Theodor Mommsen's famous quote:"It was through Africa that Christianity became the religion of the world. Tertullian and Cyprian were from Carthage, Arnobius from Sicca Veneria, Lactantius, and probably in like manner Minucius Felix, in spite of their Latin names, were natives of Africa, and not less so Augustine. In Africa the Church found its most zealous confessors of the faith and its most gifted defenders."No mention of Christian Ethiopia - not Alwa, Dongola or Axum. Axum has to be the most neglected empire of the classical "Orient." Axum was the first major kingdom to recognize Christianity as the official religion. King Ezana embraced Christianity even before the emperor of Rome. Constantine ordered in a decree dating to 336 CE that all Axumites (or subjects of the king of Axum) within his empire be treated as equal to Roman citizens. There were once several holy sites in northern Ethiopia where Christians from all over the eastern Mediterranean visited regularly (in fact they still believe the Ark of the Covenant is in Axum).
Axum and Rome exchanged embassies and were quite close for kingdoms so distant from each other. Constantine is credited with converting the Himyarites to Christianity. Though little is said of the fact he sent an Ethiopian (Axumite) missionary. Axum had annexed Yemen but later lost control to the Jewish king. This same Himyarite king then decided to slaughter some 20,000 Christians, prompting Byzantine Emperor Justin I to request then King of Axum, Kaleb, reconquer Yemen - which he did. But Christianity would never again have a strong presence on the Arabian peninsula.
Axum eventually lost control of Yemen. Their naval and trading power diminished as the Persians threw their weight around the region. There was still quite the religious convergence in northern Ethiopia. When it came to Christian Nubia the Mohammedan's posture towards non-believers was totally different. Christian Axum granted Muhammad and his followers sanctuary when they fled persecution in Arabia. They would then defeat the Muslims in two major battles following the death of Muhammad, forcing the Caliph to forge a special treaty ("bakt") which remained in tact for some 600 years. Here's what a Himyarite sheik wrote to Muhammad ibn Sa'd:"I have been to Nubia twice during the caliphate of 'Umar ibn-al-Khattab, and I never saw a people who are sharper in war-fare than they. I heard one of them say to the Moslem, ' Where do you want me to hit you with my arrow?' and in case the Moslem would disdainfully say, ' In such a spot ', the Nubian would never miss it. They were fond of fighting with arrows; but their arrows would scarcely ever hit on the ground. One day, they arrayed themselves against us and we were desirous to carry the conflict with the sword; but they were too quick for us and shot their arrows, putting out our eyes. The eyes that were put out numbered 100. We at last thought that the best thing to do with such a people was to make peace. We could carry very little booty away from them; and their ability to inflict injury was great. 'Amr, however, refused to make peace with them and went on contending against them until he was dismissed and was succeeded by 'Abdallah ibn-Sa'd ibn-abi-Sari, who concluded peace with them." (Balādhurī, 1916, pp. 380)I can think of no other region more consequential to the world's three "major" religions. For a kingdom that minted its own coins and could intervene on the behalf of Christians virtually anywhere in the region (up until the times of Saladin), Christian Nubia receives very little due.
when the Ethiopians, splendid in the
blackness of their skins and in the vigor and strength of their superb bodies, attacked with their spears, bows, and arrows they spread an unimaginable terror around them." When Ukaym says : On the day of Yathrib we were the stallions of the Arabs. It is a reference to Musrif ibn Uqba al Murri the general who gave the conquered city (Medina) over to the troops for pillage and the Negroes cohabited with the captured women, which are mentioned in the following verses of Mudar; ........
On this occasion the Zanj fought you, Whites, in spite of your rage. Wahrig defended you with his Persians, whilst the Ethiopian general commanded in the midst of destruction. ....... When the poet says: They are muleteers, assembled from everywhere, gathered as a net gathers fish in the stormy sea. He here accepted what story tellers say about Himyar.-That they used to be muleteers. The Negroes can also be proud of the fact that the single dead person over whom the Prophet ever prayed was their ruler, the Emperor of Ethiopia. He prayed for the Negus, while the Prophet was in Medina, and the tomb of the Negus in Abyssinia.