The back story is revealed as the book progresses, and is also elaborated upon in the Appendices and in The Silmarillion, the latter published after Tolkien's death. It begins thousands of years before the action in the book, with the rise of the eponymous Lord of the Rings, the Dark Lord

Sauron, a malevolent incarnated immortal spiritual being possessed of great supernatural powers, later the ruler of the dreaded realm of Mordor. At the end of the First Age of Middle-earth, Sauron survived the catastrophic defeat and exile of his master, the ultimate evil figure, Morgoth, and during the Second Age Sauron schemed to gain dominion over Middle-earth. In the guise of "Annatar" or Lord of Gifts, he aided the Elven-smiths of Eregion in the forging of magical rings which conferred various powers and effects on their wearers. The most important of these were the nineteen Rings of Power or Great Rings.
He then secretly forged a Great Ring of his own, the One Ring, by which he planned to enslave the wearers of the other Rings of Power. This plan partly failed because the Elves became aware of him and removed their Rings. Sauron then launched a war during which he captured sixteen of the Rings of Power and distributed these to lords and kings of Dwarves and Men; these Rings were known as the Seven and the Nine respectively. The Dwarf-lords proved too tough to enslave, although their natural desire for wealth, especially gold, increased; this brought more conflict between them and other races. The Men who possessed the Nine were slowly corrupted over time and eventually became the undead Nazgûl or Ringwraiths, Sauron's most feared servants. Sauron failed to capture the remaining Three, and so they remained in the possession of the Elves (Celebrimbor, leader of the Elven-smiths, had forged them independently of Sauron). The war ended as the Men of the great island-nation of Númenor helped the besieged Elves, and Sauron's forces retreated.
Over 1,500 years later, the Númenóreans sent a great force to overthrow Sauron, led by their powerful monarch Ar-Pharazôn the Golden. Deserted by his minions, Sauron surrendered and

was taken to Númenor as a prisoner. However, with cunning and strength of will he began to counsel the King, and poisoned the minds of the Númenóreans against the Valar — angelic beings who created the world. He deceived their King into invading the Undying Lands, the home of the Valar, to gain the immortality of the Elves. But upon reaching their destination, the King and his army were buried by a landslide. The Valar called upon "the One" (God), who opened a great chasm in the sea, destroying Númenor, and removing the Undying Lands from the mortal world. Sauron's fair physical body was destroyed, but his spirit returned to Mordor and assumed a new, terrible form. Some Númenóreans (called the Faithful, for they did not join the expedition) also managed to escape to Middle-earth. They were led by Elendil and his sons Isildur and Anárion.
Over 100 years later, Sauron launched an attack against the Númenórean exiles. Elendil formed the Last Alliance of Elves and Men with the Elven-king Gil-galad. They marched against Mordor, defeating Sauron's armies on the plain of Dagorlad, and besieging his stronghold Barad-dûr, at which time Anárion was slain. After seven years of siege, Sauron himself was forced to come forth and engage in single combat with the leaders. Gil-galad and Elendil were killed as they fought with Sauron, and Elendil's sword, Narsil, broke beneath him. Sauron's body was also overcome and slain,[10] and Isildur cut the One Ring from Sauron's hand with the

hilt-shard of Narsil; when this happened, Sauron's spirit fled and did not reappear for many centuries. Isildur was advised to destroy the One Ring by casting it into the volcanic Mount Doom where it was forged but, attracted to its beauty, he refused and kept it as weregild (compensation) for the deaths of his father and brother.
So began the Third Age of Middle-earth. Two years later, Isildur and his soldiers were ambushed by a band of Orcs at what was eventually called the Disaster of the Gladden Fields. The men were almost all killed, but Isildur escaped by putting on the Ring — which made mortal wearers invisible. But the Ring betrayed its wearer, slipping from his finger while he was swimming in the great River Anduin; he was seen and shot by orcs, and the Ring was lost for two millennia.
It was then found by chance by a river hobbit named Déagol. His relative and friend[10] Sméagol strangled him for the Ring and was banished from his home. Sméagol fled to the Misty Mountains where he slowly withered and became a loathsome, slimy creature called Gollum.
In The Hobbit, set 60 years before the events in The Lord of the Rings, Tolkien related the story of the seemingly accidental finding of the Ring by another hobbit, Bilbo Baggins, who took it to his home, Bag End. The tale related in The Hobbit was written before The Lord of the Rings, and it was only later that the author developed Bilbo's magic ring into the "One Ring". Neither Bilbo nor the wizard Gandalf were aware at this point that Bilbo's magic ring was the One Ring, forged by the Dark Lord Sauron.