Dagor Bragollach (455 Y.S) - The long peace was finally broken by Morgoth when he burst forth from Angband and covered the land with his dark forces.

"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...Then suddenly Morgoth sent forth great rivers of flame that ran down swifter than Balrogs from Thangorodrim, and poured over all the plain; and the Mountains of Iron belched forth fires of many poisonous hues, and the flame of them stank upon the air, and was deadly.  Thus Ard-galen perished, and the fire devoured its grasses; and it became a burned and desolate waste, full of choking dust, barren and lifeless.  Thereafter its name was changed, and it was called Anfauglith, the Gasping Dust.  Many charred bones had there their roofless grave; for many of the Noldor perished in that burning, who were caught by the running flame and could not fly to the hills.  The heights of Dorthonion and Ered Wethrin held back the fiery torrents, for their woods upon the slopes that looked towards Angband were all kindled, and the smoke wrought confusion among the defenders.  Thus began the fourth of the great battles, Dagor Bragollach, Battle of Sudden Flame." ~The Silmarillion, chap. 18 (Of the Ruin of Beleriand)