The Battle of the Gwathló was the turning point in the War of the Elves and Sauron; in 1700 SA, the Dark Lord’s forces overran Eriador, reaching as far as the River Lhûn. Aiding the Elves of Lindon was Tar-Minastir of Númenor, who sent a great force in aid, which helped to push Sauron back into the south.
As he retreated towards the River Gwathló (Greyflood), Sauron found reinforcements in Tharbad; however, they were ambushed by a second force of Númenoreans. Part of the Númenorean fleet had been sent southwards, lying in wait for Sauron. When these two armies met at the Battle of the Gwathló, Sauron was heavily defeated, and nearly captured. Though the Númenoreans had freed Eriador from Sauron, they had also earned his hatred.
Sauron was left crippled after this defeat, and through the ages slowly rebuilt himself until the thirty-third century, when he challenged Ar-Pharazôn.