Before the rising of the Sun or the Moon, Melkor Morgoth destroyed the Two Trees of Valinor and stole the Jewels of Fëanor, the Silmarils, from the vaults of their maker in Valinor. He escaped into Middle-earth, and Fëanor led his people, the Noldor, out of Valinor on a quest for revenge on the great thief.
Through hardship and betrayal, they made their way at last into the coastlands north of Beleriand, and they spread throughout that land. Fëanor himself was slain by Balrogs soon after his arrival, but his half-brother Fingolfin took up the Kingship of the Noldor and led them in a war against Melkor that would last until the end of the Age.
For much of that time, the Noldor held Morgoth besieged in his northern fortress of Angband, so that Beleriand and its surrounding lands had relative peace for a long while. At last, and without warning, Morgoth unleashed rivers of fire from his underground lair, and vast armies came down from the north to break the Siege of Angband. This was the battle known as the Dagor Bragollach , the Battle of Sudden Flame, and from this time on the lands of the Noldor were constantly at war.
Faced with such an immense force, the princes of the Noldor forged a pact, the Union of Maedhros, and joined their forces together to deal Morgoth a final defeat. The resulting battle was the greatest that Middle-earth had ever seen, and the Elves and their allies came close to success. In the end, though, they were betrayed, and Morgoth turned the tide to gain a final, apparently irrevocable victory of his own. Not for nothing was that battle called the Nirnaeth Arnoediad, the Unnumbered Tears.
All seemed lost, but through the power of one of Fëanor's Jewels, a traveller named Eärendil was able to make his way across the Great Sea to the land of Valinor. There, he pleaded the case of Elves and Men before the Valar themselves, and moved them to pity. Unforeseen by any in Middle-earth, a vast army came suddenly out of the West against Morgoth, turning his absolute victory into absolute defeat. Morgoth himself was cast out of the World until its end.
The Jewels themselves, the subject of the Age of war, were recovered. One was borne by Eärendil as he flew above Valinor in the West, and the others were taken by Maedhros and Maglor, the last survivors of the Sons of Fëanor. Both had committed evil deeds during the war, and so they could no longer touch the hallowed Silmarils. After some six hundred years of warfare to recover them, the brothers cast away the Jewels of their father in despair, one into the depths of the earth, and the other into the Sea.