Last Alliance
The Last Alliance of Elves and Men was the great military alliance formed in the Second Age to oppose Sauron after his forging of the One Ring. Its history, as recounted by Elrond at the Council, establishes the central backstory of the Ring's survival and the burden passed to the Third Age.
Origins and Formation
Elrond spoke of Sauron and the Rings of Power, and their forging in the Second Age of the world long ago. He told of the Elven-smiths of Eregion and their friendship with Moria, and their eagerness for knowledge, by which Sauron ensnared them. In that time he was not yet evil to behold, and they received his aid and grew mighty in craft, whereas he learned all their secrets, and betrayed them, and forged secretly in the Mountain of Fire the One Ring to be their master. But Celebrimbor was aware of him, and hid the Three which he had made; and there was war, and the land was laid waste, and the gate of Moria was shut.
Of Númenor he spoke, its glory and its fall, and the return of the Kings of Men to Middle-earth out of the deeps of the Sea, borne upon the wings of storm. Then Elendil the Tall and his mighty sons, Isildur and Anárion, became great lords; and the North-realm they made in Arnor, and the South-realm in Gondor above the mouths of Anduin. But Sauron of Mordor assailed them, and they made the Last Alliance of Elves and Men, and the hosts of Gil-galad and Elendil were mustered in Arnor.
The Great War and Its Climax
Elrond paused a while and sighed, recalling the splendour of their banners. It recalled to him the glory of the Elder Days and the hosts of Beleriand, so many great princes and captains were assembled. And yet not so many, nor so fair, as when Thangorodrim was broken, and the Elves deemed that evil was ended for ever, and it was not so.
Elrond was the herald of Gil-galad and marched with his host. He was at the Battle of Dagorlad before the Black Gate of Mordor, where they had the mastery: for the Spear of Gil-galad and the Sword of Elendil, Aeglos and Narsil, none could withstand. He beheld the last combat on the slopes of Orodruin, where Gil-galad died, and Elendil fell, and Narsil broke beneath him; but Sauron himself was overthrown, and Isildur cut the Ring from his hand with the hilt-shard of his father's sword, and took it for his own.
The Fateful Aftermath
Boromir broke in, crying that if ever such a tale was told in the South, it has long been forgotten. He had heard of the Great Ring of him that they do not name; but they believed that it perished from the world in the ruin of his first realm. Isildur took it! That was tidings indeed.
'Alas! yes,' said Elrond. 'Isildur took it, as should not have been. It should have been cast then into Orodruin's fire nigh at hand where it was made. But few marked what Isildur did. He alone stood by his father in that last mortal contest; and by Gil-galad only Círdan stood, and I. But Isildur would not listen to our counsel.'
Legacy and Echoes in the Third Age
The Last Alliance's failure to destroy the Ring set the stage for the entire Third Age conflict. The memory of the alliance and its incomplete victory haunted the Free Peoples, as the Ring survived to corrupt and bring war again. The alliance's legacy was felt in the bonds between Elves and Men, and in the very weapons and realms that endured. The mallorn-leaf of Lórien and the orc-weapon found later by Aragorn, Gimli, and Legolas were reminders of the long shadow cast by that ancient war, as the quest to undo Isildur's error unfolded. The fall of the Dark Tower and the destruction of Isengard in the War of the Ring finally completed what the Last Alliance had left undone, fulfilling the promise that the Shadow is a passing thing.