The Battle of the Plains was fought between the Men of Rhovanion and the Gondorians, against invading Wainriders.

The Wainriders were victorious, and Narmacil II of Gondor was killed, as was Marhari of the Northmen. After the battle, all of Gondor's eastern realms fell under the control of the Wainriders, who remained a constant threat for nearly another century.
Encyclopedia entry originally written by atalante_star