The Battle of Callinicum took place on Easter Saturday, 19 April 531 AD, between an army of the Byzantine Empire under Belisarius and a Sasanian cavalry force commanded by Azarethes. After being defeated at the Battle of Dara, the Sasanians moved to invade Roman Syria in an attempt to turn the tide of the war. Belisarius' rapid response foiled the plan, and his troops pushed the Persians to the Syrian border through maneuvering before forcing a battle in which the Sasanians won a Pyrrhic victory.
Prelude
In April 531 AD, the Persian king Kavadh I sent an army under Azarethes, consisting of a cavalry force numbering about 15,000 Aswaran with an additional 5,000 Lakhmid Arab cavalry in order to capture Syrian cities such as Antioch.
The Persian army crossed the frontier at Circesium on the Euphrates and marched north. As they neared Callinicum, Belisarius set out to follow them as they advanced westwards. Belisarius' forces consisted of about 5,000 men and another 3,000 Ghassanid Arab allies, for the remainder of his army had been left to secure Dara. The Byzantines blocked the Persian advance at Chalcis, where reinforcements under Hermogenes also arrived, bringing the Byzantine force to some 20,000 men. The Persians were forced to withdraw to the east, followed by the Byzantines. In spite of Belisarius orders, Sunicas' forces successfully raided the Persian army near Gabbulon. Hermogenes was also inclined to harass the Persian retreat; this attitude was apparently motivated by the killing of captives by Al-Mundhir when his army ravaged Syria in 529. Azarethes sent a message to Belisarius asking him to honor the fast for the sake of the Christians and Jews in his army, as well as the Christian forces of Belisarius himself. Belisarius was willing to agree and originally intended to drive off the Persians without a risky battle. The Byzantine troops, however, were over-confident after their recent victories at Dara and Satala and clamored for battle. After failing to convince his men, and realizing they would possibly mutiny unless he agreed, Belisarius prepared his force for battle.
Deployment
The two armies met outside Callinicum on 19 April 531. Belisarius anchored his left flank on the Euphrates, composed of the Byzantine heavy infantry under the command of the emperor Justinian's bodyguard Petrus. The Byzantine cavalry was stationed in the center, many of which were cataphracts under the command of Ascan. Next were the Lycaonian and/or Isaurian infantry under Stephanacius and Longinus, positioned such that their right was anchored on a rising slope occupied by the army's right wing, which consisted of the 5,000 allied Ghassanid cavalry. Belisarius himself took up a position in the center of his army. Unlike his deployment at Dara, he concentrated his best cavalry in the center and the infantry and allied cavalry on the wings, possibly because he anticipated the infantry and Ghassanid cavalry could flee. It is possible that he also held a reserve of elite Aswaran behind his center.
Battle
thumb|300px|Azarethes reinforces his left wing and the Byzantine right wing is beaten back.
thumb|300px|The Byzantine center, including the cavalry, is exposed and routed. Infantry led by Petrus delay the Persian advance for some defenders to retreat.
The battle began with extensive exchange of arrow shots,
Later, after "two-thirds of the day" had elapsed, Azarethes found a weak spot in the Byzantine force and redeployed some of his cavalry to his left wing; this was a similar tactic used at Dara, where the Persians tried to find a weak spot and overwhelm it by creating local numerical superiority. This maneuver was not observed by Belisarius and proved to be a turning point. The Ghassanids were routed off the field with such ease as to later inspire accusations of treachery. This exposed the right flank of the Lycaonian infantry, who were no match for the Persian cavalry and were also routed, while their commanders were killed. The right flank of Ascan's heavy cavalry was now exposed; they fought as best they could, but Ascan was killed and his force defeated.
Zacharias of Mytilene said of the battle: "[The Romans] turned and fled before the Persian attack. Many fell into the Euphrates and were drowned, and others were killed." However, it is unknown what stage of the battle Zachariah was referring to.
The factuality of Procopius's description of the events in this battle has been questioned.