Second Battle of Dagorlad
This article describes a concept which is mentioned in J.R.R. Tolkien's works, but was never given a definite name.
An Easterling people known as the Wainriders began attacking Gondor. Gondorians and Northmen who moved against them were defeated in the Battle of the Plains (T.A. 1856). Most Northmen lands were occupied by the Wainraiders and were enslaved, however other escaped and some followed Marhwini to the Vales of Anduin.
Years later, the Wainriders planned to raid Calenardhon across the Undeeps. Lord Marhwini sent message to King Calimehtar warning him about this, notifying him also that the enslaved Northmen were also preparing a revolt should the Wainriders become involved in war.
 The Battle
While moving his army, Calimehtar did what he could to make the enemies know of his movements in order to lure them. Indeed, gathering all of their available strength, the Wainriders came down to meet him and the King gave way before them, drawing them away from their homes, leaving the Undeeps unguarded.
The two forces met on Dagorlad where battle developed. All seemed in doubt, until at the peak of the battle, upon the flank and rear of the Wainriders, fell Calimehtar's horsemen from the Undeeps, who were joined with a great éored led by Marhwini. The joint army drove the Wainriders back to the East.
After the battle, desperate Northmen who became outlaws came out of Mirkwood and roused the slaves of the Wainriders. They burned many of their enemies' dwellings, storehouses and fortified camps of wagons. Most of them perished in the attempt fighting the Wainriders' youths, women and old men.
Calimehtar's and Marhwini's victory was overwhelming and Gondor enjoyed a respite from war for 45 years. Marhwini retired to his land beside the Anduin, and the Northmen of his race never again returned to their former homes.