The Field of Celebrant was where the fierce Easterlings known as the Balchoth waged open war against a weakened Gondor, and had already overran most of Wilderland. Gondor had no allies left east of Anduin, and against hope had sent the Red Arrow north to the Éothéod, ancient allies of Gondor, but they had received no reply.
In 2510 the Balchoth crossed Anduin at numerous points, passing into the Wold in the north of the Gondorian province of Calenardhon. They met little resistance as Calenardhon had been mostly abandoned long before, and by the time the North Army of Gondor appeared from the fortresses of Isengard, Aglarond and other fortresses in the White Mountains the bulk of the Balchoth army had crossed. The North Army counter-attacked, and was driven into the Wold, cut off from the later reinforcements by the Balchoth host. By the time the South Army appeared, the North Army had come under attack by an orcs band which by chance or design had descended from the mountains, and the Dúnedain were backed against the river in a hopeless situation.
At this time the Éothéod under their leader Eorl the Young suddenly appeared, unexpected by friend or foe. The Éothéod crossed the Anduin at the Undeeps of the river, and broke on the rearguard of the Balchoth. The Balchoth were completely defeated and the Gondorian armies saved. The Éothéod continued their foray into northern Gondor, scattering and destroying all the Balchoth in Calenardhon.