§ Of the Fifth Battle: Nirnaeth Arnoediad The Silmarillion

But Maedhros made trial of his strength too soon, ere his plans were full-wrought; and though the Orcs were driven out of all the northward regions of Beleriand, and even Dorthonion was freed for a while, Morgoth was warned of the uprising of the Eldar and the Elf-friends, and took counsel against them. Many spies and workers of treason he sent forth among them, as he was the better able now to do, for the faithless Men of his secret allegiance were yet deep in the secrets of the sons of Fëanor.