The Wainriders were beginning to recover from their defeat at Dagorlad and were apparently not completely driven out by the rebellion in Rhovanion. Following this their power expanded east and south of Mordor and they gained new allies in Khand and Harad. The new Northmen lord, Forthwini discovered a new attack was being planned and warned King Ondoher of it. Meanwhile, Eärnil, based at Pelargir received this news on the ninth day of Cermië and gathered a force that crossed the river Anduin in part, leaving the Fords of the Poros purposely undefended.
Eärnil's southern army camped forty miles north of the Poros in South Ithilien apparently waiting to face the enemy from the south or to support the forces led by his king in the north. Unfortunately, the enemy attacked at the same time and things went ill for the king at the Black Gate, leading to his death and those of his sons. Now Eärnil would face an army of Haradrim.
The Haradrim crossed the Poros and Eärnil met them in battle. Eärnil utterly defeated them destroyed the entire army. Little to no details are known about the battle itself save that the Haradrim army was completely destroyed, and Eärnil's losses were apparently light.
After Eärnil's victory, he and his army moved north to assail the Wainriders, who believing they were now victorious over Gondor were busy feasting and celebrating in their camp. Here, Eärnil would defeat them once and for all.
- Unfinished Tales: Cirion and Eorl and the Friendship of Gondor and Rohan, The Northmen and the Wainriders
- The Atlas of Middle-earth pgs. 58-9
- The Lord of the Rings: Appendix B, "The Third Age"