Union of Maedhros
- Eastern army
- Elves of Himring (Maedhros)
- Easterlings under the service of Maedhros ((Bór)
- Easterlings under the service of Caranthir (Ulfang)
- Dwarves of Belegost (Azaghâl)
- Dwarves of Nogrod
- Western army
- Noldor of Hithlum (Fingon)
- Edain of Dor-lómin (Húrin and Huor)
- Edain of Brethil (Haldir)
- Elves of Nargothrond (Gwindor)
- Some Falathrim
- Mablung and Beleg from Doriath
After the Dagor Bragollach, the Noldor had lost control over the northern Beleriand, and were mostly reduced to enclaves in Hithlum and Nargothrond, including the hidden Gondolin. When Maedhros heard of the heroic deeds of Beren and Lúthien new hope came to him. Maedhros decided to unite the peoples of Beleriand to assail Angband.
Elves, Men and Dwarves joined the alliance. The Dwarves of the Blue Mountains supplied both armed forces and forged weapons. Even to Gondolin came word of the Union and Turgon the Hidden King secretly made his own preparations. Other Elves, like Orodreth, refused to join because of the evils of Celegorm and Curufin, while Thingol had sworn never to fight alongside the Feanorians; only a few representatives came from Doriath and Nargothrond.
Maedhros' plans were to lead his army (with some assistance from the western army) to Anfauglith in order to draw out the armies of Morgoth, and then flank with the hidden forces of Fingon that would emerge from the Ered Wethrin.
Initially the forces managed to clean of evil creatures West Beleriand and Dorthonion, and the final assault was planned for Midsummer. However Morgoth had time to prepare his forces as Maedhros revealed himself too early, and then Uldor, who was an agent of the Enemy, delayed Maedhros' forces by giving false reports of enemy presence. A small force of Orcs managed to provoke the western army prematurely, instigated by Gwindor. As a result, the armies of the Union were themselves outflanked in the ensuing battle, which, despite Turgon's timely and unexpected arrival, would prove disastrous, and would be known as the Nirnaeth Arnoediad.