In 537 AD, Cottistis was serving in the Byzantine army as an infantry soldier. He was stationed at Dara, where he started a military revolt against Emperor Justinian I (r. 527–565). He was likely declared emperor, because the sources describe his as a "tyrant", which was the term often used for usurper emperors. He was killed in battle four days following the start of his rebellion. According to the 6th-century historian Procopius: "In the city of Daras the following event took place. There was a certain John there serving in a detachment of infantry; this man, in conspiracy with some few of the soldiers, but not all, took possession of the city, essaying to make himself tyrant. Then he established himself in a palace as if in a citadel, and was strengthening his tyranny every day. And if it had not happened that the Persians were continuing to keep peace with the Romans, irreparable harm would have come from this affair to the Romans. But as it was, this was prevented by the agreement which had already been reached, as I have said." The agreement mentioned by Procopius was the Eternal Peace of 532, which was signed at the end of the Iberian War. The rebellion took place during a period of cooperation between the Byzantine and Sassanid Empires, the two great powers of the Middle East. Justinian was focusing his resources on his wars of reconquest against the Vandals and in Italy against the Goths, neglecting the defences of the Byzantine Empire in the East.