The Battle of the Gwathló was the battle that removed Sauron from Eriador, which he had nearly conquered, in SA 1700.

During the battle, Sauron’s army had reached the River Lhûn when he summoned more forces from the southeast, which by that time had reached Tharbad. The Dark Lord also sent a strong detachment to contain Rivendell and keep Elrond and his army from attacking the rear of Sauron’s. Gil-galad and his army were left desperately defending the Grey Havens.

Gil-galad had called upon Númenor for aid five years before, in SA 1695, and Tar-Minastir (the king at that time) had sent a large fleet. But The Númenóreans were delayed and didn’t arrive until the Battle of the Gwathló.

When Ciryatur (the Númenórean admiral) arrived, he sent some of his ships south to Lond Daer at the mouth of the River Gwathló-Mitheithel (also called the River Greyflood).

The Númenóreans greatly assisted Gil-galad and Sauron was driven southeast, where he joined with the part of his army in Tharbad. Though he was strengthened by those there, the Númenóreans came up upon the rear of Sauron's army and he was almost utterly defeated. He returned then to Mordor.

Meanwhile, the armies of Lindon and Rivendell had crushed the remainder of Sauron’s army that had been besieging the Grey Havens. Eriador was freed, but was largely damaged.
Encyclopedia entry originally written by LadyEowyn_Of_Rohan