In J. R. R. Tolkien's fictional Middle-earth, as told in the Silmarillion, the Dagor Aglareb ([ˈdɑɡɒr ˈɑɡlɑrɛb]) was the third battle of the Wars of Beleriand during the First Age, known as the Glorious Battle.
Following their victory in Dagor-nuin-Giliath, the Second Battle, the Noldor of Beleriand let pass their chance to defeat Morgoth quickly by following up and storming Thangorodrim. Instead they established and built their numerous realms, mingling with the Sindar, while watching Angband.
After a time of rebuilding and reorganizing his armies, Morgoth thought to catch the Noldor off guard and test their strength. Under cover of fire and smoke spewing forth from Ered Engrin, the Mountains of Iron, Morgoth sent forth an Orc-host against Dorthonion flanked east and west by armies of numerous small bands of Orcs. The flanking armies advanced through the Pass of Sirion and Maglor's Gap in an attempt to draw the principal Noldorin forces under Fingolfin and Maedhros away from the attack of the main host on Finrod's brothers, Angrod and Aegnor, who held Dorthonion.
In the event, Morgoth's diversionary attacks failed their purpose to divert the main strength of the Noldor. Fighting small battles along the way, the Orcs penetrated through the Pass of Sirion into West Beleriand but were rooted out and hunted down by Finrod and Turgon. In East Beleriand, they were defeated, pursued and destroyed by Maglor and southern forces of the Noldor led by Amrod, Amras and Caranthir.
The main Orc-host assaulted Dorthonion, where Angrod and Aegnor held them off until armies under Fingolfin and Maedhros counter-attacked on the plains of Ard-galen, trapping the Orcs between them. The Orcs were pursued by the Noldorin cavalry as they retreated to Thangorodrim and were utterly destroyed in sight of the gates of Angband.
Morgoth's strategy was to divert the main strength of the Noldor from Dorthonion with attacks on its flanks. The wooded highlands of Dorthonion were the true object of his campaign. By capturing Dorthonion, he could establish a defensible forward base on high ground which divided the Noldor of Fingolfin from those of Maedhros. This strategy failed and it was because of this defeat that Morgoth realised that Orc-captains and Orc-armies were insufficient. He began to consider other more powerful leadership and auxiliaries. This was a complete victory by the Noldor over Morgoth. After it the Noldor maintained a guard on Angband, beginning the period of watchful peace known as the Siege of Angband. The guard consisted of building forts on the hills and mountains west, south and east of the plains south of Angband and maintaining forward camps of cavalry on the plains of Ard-galen and Lothlann.