After five years of ravaging eastern Gondor and the Northmen of Rhovanion, Gondor assembled a large army with many scattered remnants of loyal Northman allies under the leadership of Narmacil II. The King marched his great host north from Ithilien to the open plains just south of Mirkwood where they met their foe. However, Narmacil's great host was defeated and cost Narmacil his life.
After the battle was lost, the Northman noble Marhari involved in leading a rearguard action against the enemy was slain. The survivors fled south over Dagorlad and back into Ithilien and Gondor was forced to abandon its eastern lands. The majority of the Northmen fell into slavery and those that did not withdrew further north over the River Running and into Dale, where they joined with the Men of Dale. Others gathered around Marhari's son Marhwini, who settled the Vales of Anduin between Mirkwood and the river Anduin with fugitives slaves from Rhovanion, who joined with them later. These survivors and refugees would later become the Éothéod, an ally of Gondor.
The loss at the Plains led to the later creation of the Rohirrim out of the Éothéod who were survivors of the battle. These warrior-folk would maintain friendship with Gondor and centuries later would take the Oath of Eorl and settle the Gondor province of Calenardhon, later renamed Rohan and remain an eternal ally of Gondor and the later Reunited Kingdom, fortunately, there foes suffered from not light casualties as well, providing enough time for them to strike back.