As the years drew on after the Return of the Noldor to Middle-earth, they seemed content to leave Morgoth quiet in Angband, setting no siege and attempting no attack of their own. Concluding that his enemies were unprepared for war, Morgoth resolved to launch a sudden assault of his own: Orcs by the thousand marched south from Angband's gates, while the earth shook and the Iron Mountains burst into flame.
This was one of the greatest victories of the Elves over Morgoth, and it became known as the Glorious Battle, or Dagor Aglareb. It also served as a warning, and the Eldar redoubled their defences against Angband. After the Dagor Aglareb, they set the great Siege of Angband, and the people of Beleriand were kept safe from any major attack until the time of the Dagor Bragollach, nearly four centuries later.