§ Of the Ruin of Beleriand and the Fall of Fingolfin The Silmarillion

But when the sixth generation of Men after Bëor and Marach were not yet come to full manhood, it being then four hundred years and five and fifty since the coming of Fingolfin, the evil befell that he had long dreaded, and yet more dire and sudden than his darkest fear. For Morgoth had long prepared his force in secret, while ever the malice of his heart grew greater, and his hatred of the Noldor more bitter; and he desired not only to end his foes but to destroy also and defile the lands that they had taken and made fair. And it is said that his hate overcame his counsel, so that if he had but endured to wait longer, until his designs were full, then the Noldor would have perished utterly. But on his part he esteemed too lightly the valour of the Elves, and of Men he took yet no account.