Parthian king Vologases IV enters the Kingdom of Armenia (a Roman client state) in the late summer autumn of 161, expels its king and installs his own—Pacorus, an Arsacid like himself.

At the time of the invasion, the governor of Syria is L. Attidius Cornelianus, who had been retained as governor even though his term ended in 161, presumably to avoid giving the Parthians the chance to wrong-foot his replacement.

The governor of Cappadocia, the front-line in all Armenian conflicts, is Marcus Sedatius Severianus, a Gaul with much experience in military matters.

Persuaded by Alexander of Abonutichus, a prophet who carries a snake named Glycon around with him, that he can defeat the Parthians easily, and win glory for himself, Severianus leads a legion (perhaps the IX Hispana) into Armenia, but is trapped by the great Parthian general Chosrhoes at Elegia, a town just beyond the Cappadocian frontiers, high up past the headwaters of the Euphrates.

Severianus makes some attempt to fight Chosrhoes, but soon realizes the futility of his campaign, and commits suicide.

His legion is massacred.

The campaign had only lasted three days.