War of the Last Alliance

The Last Alliance of Elves and Men was a great alliance formed in 3430 of the Second Age in response to the threat of conquest by the Dark Lord Sauron.

This conjoined army of Elves, under Gil-Galad the Elvenking and the wodland realm under king Oropher met with King Elendil's host which was the largest force ever assembled in Middle-earth since the Host of Valinor during the War of Wrath. The Last Alliance was the greatest military alliance involving all the Free Peoples of the world united under one banner, and the only large-scale war fought until the days of the climax of the War of the Ring. After this war, the Elven people never managed to bring about such a great and mighty show of force in war again. The Elven people later left Middle-earth for Valinor.

History
Because Elendil and Gil-galad had taken the initiative, they had the luxury of time to prepare for the Battle of Dagorlad. Both kings spent two years gathering their forces; Elendil and Isildur met Gil-galad at the watchtower of Amon Sûl on Weathertop and together they led their hosts to Rivendell, where they were joined by Elrond and his company of Elves. In Rivendell they spent a further three years making plans, forging weapons, and training for the great battle ahead of them. That is the reason Rivendell is also known as The Great Forge.

They journeyed south down the River Anduin, gathering Elves led by Oropher and Amdír, the kings of Greenwood and Lórien, respectively, and eventually were joined by the army of Númenóreans from Gondor led by Isildur's brother Anárion. But when they reached the area of Fangorn forest, a battle took place. Sauron had sent a host ahead of them to destroy the Entwives who lived there. From then on, they were believed to be wiped out The host that comprised the Last Alliance of Elves and Men is said to have been the greatest seen in Middle-earth since the War of Wrath. It consisted of hundreds of thousands of Elves and Men.

The greatest and most decisive battle took place before Mordor, on the plains of Dagorlad. It was here when the Last Alliance destroyed Sauron's main force. Although this was a victory, many were lost, including both kings Amdír and Oropher due to their annoyance at following the supreme command of Gil-galad, the High King of the Ñoldor, and thus charging prematurely at the enemy. Many Silvan Elves were also killed alongside Oropher, and so were many Elves and Men. Oropher's army fell in the face of the Black Gate but Amdír and his army were driven back all the way to the Dead Marshes where they were slain bitterly. the bodies of these elves stayed preserved in these Marshes up until Frodo and Sam passed it with Gollum.

After this battle, the Black Gate was thrown down by the host of men and Elves who followed Gil Galad and Elendil and the Siege of Barad-dûr began, lasting seven years, during which Anarion, son of Elendil and lord of Minas Anor, was killed by a rock shot from a catapult atop Barad-dûr. It is said that Anarion and his army had made it all the way to the gates of Barad-dûr without the help of the other armies but as soon as Anarion fell, the army joined the main host and came to the command of Elendil. The war ended when Sauron himself came forth and fought Gil-galad in combat, who was then joined by Elendil. Although they two both fell, they managed to defeat Sauron and bring him down. Isildur in the same hour then subsequently cut the One Ring from Sauron's hand, dispersing his spirit.

It should be noted that while it is called the Last Alliance of Elves and Men, Tolkien states that all races came to battle that day, even birds and beasts, and that all were found on either side except for the Elves, who fought solely under the banner of Gil-galad.

Aftermath
After this defeat of Sauron, several notable things occurred. The battle brought a close to the Second Age and began the Third Age. The High Kingship of the Dúnedain passed to Isildur, son of Elendil; however the death of Gil-galad ended the line of High Kings of the Ñoldor and the titles of King of Lindon and High King of the Elves in Middle Earth ceased to be because no one had the authority to claim them. Also, Thranduil and Amroth became the kings of Greenwood and Lothlorien, respectively, after the deaths of their fathers Oropher and Amdír.

Three years after the war ended, Isildur was killed at the Gladden Fields, and the One Ring was lost in the river for two and a half millennia. Sauron later became the Necromancer.

Peter Jackson's The Fellowship of the Ring
Peter Jackson's The Lord of the Rings: The Fellowship of the Ring differs in some ways from the above account, supplying more details and changing some aspects. The elves used falxes (two-handed, curved swords) and Sauron wielded a giant mace (considering that the elven forces consisted of Ñoldor, the Elves should have used straight swords like the Dúnedain. In any case, it was the Ñoldor who taught men in the First Age how to craft various weapons, so the Dúnedain's weapons should be very similar to the Elven weapons). The original concept for the battle included Sauron and Gil-galad's duel, and Sauron seizing Gil-galad by the throat when the High King fell; the heat of Sauron's hand would cause the elf lord to burst into flames. However, this scene was cut.

The remainder of the scene is in the film. Elendil goes to avenge Gil-galad but a blow from Sauron's mace throws him against some rocks and he is killed. His son Isildur rushes to his side. Sauron approaches and Isildur grabs his father's sword, Narsil. However, Sauron steps on the blade and it breaks into several pieces. Isildur uses the hilt to slice off Sauron's fingers, including his right index finger, which bore the One Ring. As Sauron's life force is bound to the ring, his armor explodes and his body and spirit vanish. The force of the explosion knocks over everyone in the surrounding area.

Some important characters are never mentioned. Cirdan and Anarion are both omitted. While Gil-Galad is never identified by name he can still be spotted, recognisably the Elf who viciously skewers an orc with his spear.

In the book both Elrond and Cirdan counselled Isildur to destroy the Ring. In the film Elrond immediately takes Isildur to the Crack of Doom itself after the battle. There he urges Isildur to destroy the Ring, but Isildur refuses and departs.