War of the Elves and Sauron

(1)	Sauron marches from Mordor to Eregion, and lays waste to that land.
(2)	An army led by Elrond is driven away to the north, accompanied by the survivors of Eregion. They take refuge in a hidden valley, and thus Imladris (Rivendell) is founded; Elrond and his people are besieged there.
(3)	Sauron's western forces advance toward Lindon, reaching the river Lhûn on the borders of that land.
(4)	A Númenórean navy sent by Tar-Minastir lands in Lindon and drives Sauron back into Eriador, defeating him in battle at Sarn Ford.
(5)	A second Númenórean fleet lands at the mouths of Gwathló and intercepts Sauron's retreating forces at the Battle of the Gwathló, and the defeated Sauron flees back to Mordor.

Solid dark arrows show the routes of Sauron's advance, while the finer dashed arrows show his retreat to Mordor. Elements of this map are conjectural.¹

During the Second Age, Sauron planned to take control of the Elves through the power of his One Ring. In his guise as Annatar, he instructed the Elves of Eregion in the making of Rings of Power, and then secretly constructed a Ruling Ring of his own to enslave the bearers of the Rings. Unknown to him, the Elves had made Three Rings of their own, so that when Sauron took up his own Ring, they became aware of his malign intent, and so his centuries of planning came to nothing.

What he could not control by subtlety, Sauron resolved to destroy by force, and so began the War of the Elves and Sauron. At first, Sauron's victories came quickly: vast armies emerged from Mordor and, over the next six years, overran Eriador. Eregion was destroyed, and Sauron claimed the remaining Rings of Power (except for the Three Rings, which had been sent far from Eregion at the beginning of the war).

Just as Sauron's victory looked secure, a great navy appeared in the north, carrying the forces of Númenor sent by King Tar-Minastir. The Númenóreans put Sauron's armies to rout, and forced them back out of the Westlands. So Eriador was saved, but Celebrimbor's kingdom of Eregion had been left in ruins, and a great many of the Rings of Power had fallen into the hands of Sauron.