A comparison of the size of armies throughout history, carried out a few years ago by Slovak cartographer Martin Vargic, shows that, except for the special case of China, the number of combatants reached its peak around 500 BC and then gradually decreased, only to increase again from the 19th century onwards.
The armies of Sargon of Akkad already numbered several thousand, so it was difficult in ancient times to keep track of the total number of soldiers, and it was much more difficult to count the casualties in battle. Classic sources often contradict each other when reporting figures, exaggerating or minimizing them according to interest, but also because there were no reliable records to refer to. In fact, it was not until the 19th century that armies had effective methods for making such calculations.
On April 19, 531 A.D., two fairly evenly matched forces (about 20,000 men on each side) clashed on the banks of the Euphrates near present-day Raqqah in Syria. On one side were the Byzantine troops under the command of Belisarius, and on the other those of the Sassanian Empire led by Azarethes. The clash is known as the Battle of Callinicum and would be one of many in which the two powers would face each other.
It had all started in 527, when Persian Sasanian King Cabades (Kavadh I) tried to take control of Caucasian Iberia by forcing its inhabitants to convert to Zoroastrianism. The Byzantine Emperor Justinian I sent his armies under the command of his best generals, Sittas and Belisarius, against the Persians. Clashes ensued, with alternate victories for each side and, after a brief truce and negotiations that did not bear fruit, the Persians decided to strike the final blow.
Cabades’ army entrenched itself at Nisibis (present-day Nusaybin in southeast Turkey) while the cavalry under the command of Azarethes went up the Euphrates to raze Syria to the ground until they took Antioch. But Belisarius cut off Azarethes and pursued him to Assyria, reaching him at Callinicum. The battle was a disaster for the Byzantines, mainly due to the fleeing of their Ghassanids allies. Only the night saved them, when Belisarius ordered the survivors to swim across the river to an islet where they were picked up by ships.
With Belisarius on the battlefield was his assistant and historian Procopius of Caesarea, who witnessed the battle, and whose works are the main source of information about Justinian’s reign. In his History of Wars he recounts the battle and says:
Now when Azarethes came into the presence of the king, Cabades enquired of him whether he came back with any Roman fortress won over to their side, for he had marched forth with Alamoundaras against the Romans, with the purpose of subduing Antioch. And Azarethes said that he had captured no fortress, but that he had conquered the Romans and Belisarius in battle. So Cabades bade the army of Azarethes pass by, and from the baskets each man took out a weapon just as was customary. But since many weapons were left, Cabades rebuked Azarethes for the victory and thereafter ranked him among the most unworthy. So the victory had this conclusion for AzarethesProcopius of Caesarea, History of Wars 1, XVIII
What had happened? Very simple. Azarethes had won the battle, but had failed to conquer any cities or fortresses. Moreover, the Persian casualties had been so heavy that Azarethes was relieved of command.
And how could Cabades know how many soldiers had fallen? Procopiu’s account gives us the clue. Before each battle the entire Persian army paraded one by one in front of the king, and each man deposited an arrow or other weapon in the baskets set up for this purpose. When they returned from battle, the process was repeated, with each man taking one of the weapons from the baskets. They counted the weapons that had not been collected, and thus knew exactly how many men they had lost.
When Azarethes reached Persia with his army, although he had prospered in the battle, he found Cabades exceedingly ungrateful, for the following reason. It is a custom among the Persians that, when they are about to march against any of their foes, the king sits on the royal throne, and many baskets are set there before him; and the general also is present who is expected to lead the army against the enemy; then the army passes along before the king, one man at a time, and each of them throws one weapon into the baskets; after this they are sealed with the king’s seal and preserved; and when this army returns to Persia, each one of the soldiers takes one weapon out of the baskets. A count is then made by those whose office it is to do so of all the weapons which have not been taken by the men, and they report to the king the number of the soldiers who have not returned, and in this way it becomes evident how many have perished in the war. Thus the law has stood from of old among the PersiansProcopius of Caesarea, History of Wars 1, XVIII
As for Azarethes, Chosroes I, son and successor of Cabades, reinstated him in command and Procopius tells how he participated in the siege of Edessa in the year 544:
And the soldiers with much shouting and tumult brought up the towers and the other engines of war to the wall and set the ladders against it, in order to capture the city with one grand rush. But since the Romans were hurling great numbers of missiles and exerting all their strength to drive them off, the barbarians were turned back by force; and as Chosroes withdrew, the Romans taunted him, inviting him to come and storm the wall. Only Azarethes at the so-called Soinian Gate was still fighting with his men, at the place which they call TripurgiaProcopius of Caesarea, History of Wars 2, XXVII
Procopius himself, who survived the battle of Callinicum, would later accompany Belisarius in the conquest of Carthage from the Vandals in 533 A.D., and in the campaign against the Ostrogothic kingdom of Italy in 537:
During this winter Belisarius remained in Syracuse and Solomon in Carthage. And it came about during this year that a most dread portent took place. For the sun gave forth its light without brightness, like the moon, during this whole year, and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear nor such as it is accustomed to shed. And from the time when this thing happened men were free neither from war nor pestilence nor any other thing leading to death. And it was the time when Justinian was in the tenth year of his reignProcopius of Caesarea, History of Wars 4, XIV
He was referring to the meteorological phenomena known today as the Late Antique Little Ice Age, which took place between 535-536 A.D. caused by a large volcanic eruption in the tropics or by the impact of a meteorite.
Sources: Delphi Complete Works of Procopius (Procopius of Caesarea) / One of the last: Belisarius and the persians / Wikipedia.