Caracallus appears to have passed the winter at Edessa, amusing himself with hunting and charioteering after the fatigues of his campaign.  In the spring he threatened another advance into Parthian territory, and threw the Medes and Parthians into great alarm.  He had not, however, the opportunity of renewing his attack.  On April 8, A.D. 217, having quitted Edessa with a small retinue for the purpose of visiting a famous temple of the Moon-God near Carrhaa, he was surprised and murdered on the way by Julius Martialis, one of his guards.  His successor, Macrinus, though a Praetorian prefect, was no soldier, and would willingly have retired at once from the war.  But the passions of the Parthians had been roused.  Artahanus possessed the energy and spirit which most of the recent monarchs had lacked; and though defeated when taken at disadvantage, and unable for some months to obtain any revenge, had employed the winter in the collection of a vast army, and was determined to exact a heavy retribution for the treacherous massacre of Ctesiphon and the wanton impiety of Arbela.  He had already taken the field and conducted his troops to the neighborhood of the Roman frontier when Caracallus lost his life.  Macrinus was scarcely acknowledged emperor when he found that the Parthians were close at hand, that the frontier was crossed, and that unless a treaty could be concluded he must risk a battle.

Under these circumstances the unwarlike emperor hurriedly, sent ambassadors to the Parthian camp, with an offer to restore all the prisoners made in the late campaign as the price of peace.  Artabanus unhesitatingly rejected the overture, but at the same time informed his adversary of the terms on which he was willing to treat.  Macrinus, he said, must not only restore the prisoners, but must also consent to rebuild all the towns and castles which Caracallus had laid in ruins, must make compensation for the injury done to the tombs of the kings, and further must cede Mesopotamia to the Parthians.  It was impossible for a Roman Emperor to consent to such demands without first trying the fortune of war, and Macrinus accordingly made up his mind to fight a battle.  The Parthian prince had by this time advanced as far as Nisibis, and it was in the neighborhood of that city that the great struggle took place.

The battle of Nisibis, which terminated the long contest between Rome and Parthia, was the fiercest and best-contested which was ever fought between the rival powers.  It lasted for the space of three days.  The army of Artabanus was numerous and well-appointed:  like almost every Parthian force, it was strong in cavalry and archers; and it had moreover a novel addition of considerable importance, consisting of a corps of picked soldiers, clad in complete armor, and carrying long spears or lances, who were mounted on camels.  The Roman legionaries were supported by numerous light-armed troops, and a powerful body of Mauritanian cavalry.