The First War (also known as "First War with Melkor" or "War to begin all Wars") was a string of battles before the formation of Arda. Melkor led his rebellious Maiar against the host of the Valar. Under command of Manwe and twelve other Valar, Melkor stood against all thirteen by himself. Melkor at this time did seduce Osse and Sauron, and they served as great generals to him besides Fluithuin, Langon, Gothmog and many more Umaiar. While the Valar boasted Eonwe, Olorin, Curumo, Tilion, Alatar, Uinen, Makar, Measse, Arien, and many more Maiar as well. The wars were not recorded, but it is conceivable that Morgoth's forces were probably one third of Eru's forces.
The war went on until Valian Year 1499 where Tulkas came down and made Melkor flee Arda. The rest of his forces fled as well and hid until Melkor's return.
After returning to Middle Earth, Melkor ushered the end of the Valarin Years by ending the First War by demolishing the Two Lamps.
Battles during the First War: