The War of Trebizond was a short and undeclared (but very bloody) war between the Sassanid Empire and the Byzantine Empire. It was started by Yazdegerd's claim over his former areas in Armenia and ended after he and Emperor Constans of Byzantium both died of mortal wounds during the last phase of the war. The sides, evenly matched, caused huge casualties between both of them; however, the war is generally considered a Sassanid victory.
The Byzantine-Sassanid War of 635-650 proved fatal to the Sassanid Empire. It was forced to give up most of Arabia and the Empire's Armenian holdings to a Byzantine vassal state. Not only had Persia lost important lands, its honour was also extremely damaged.
Yazdegerd III, Emperor of Iran, thought the nation's prestige would be recuperated if he were to win against the Byzantines in another war. He soon gathered most of his remaining army (numbering about 100,000 people) and, without a formal declaration of war, marched northwest.
Occupation of Armenia and Attack on Trebizond
Armenia and Byzantium were both taken extremely by surprise. Not only had the war just ended and they expected Yazdegerd to wait a while so to recuperate, but they also expected him to march through the south. Armenia therefore fell within weeks and the Medinans declared they would be neutral in the war.
With Armenia subjugated (and annexed), Yazdegerd could have stopped; but he wished to claim the entirety of Anatolia for himself. He continued marching west, setting his sights on the fortress of Trebizond.
Constans, however, was waiting with a recently-drafted army, numbering about the same as the Persian army. A huge battle broke out in the outskirts of Trebizond. The battle, which dragged on for several days, ended up with 95% of the Byzantine army and 85% of the Sassanid one dead, among them both nations' leaders.
180,000 people lay dead, among them the leaders of both Persia and Byzantium. The Battle of Trebizond was among the largest disasters of Middle Eastern military history.
Byzantium and Persia's heirs both were underage, and it led to bureaucracy seizing power over both nations. The weakening of both nations and the establishment of the Treaty of Babylon swiftly followed.