The History of Middle Earth

The Last alliance of Men and Elves

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.

Climbing over the Misty Mountains they had started recruiting the Dwarves of Moria under the command of Durin IV. 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 until they showed themselves again. In the third age when Pippin and Merry convince them to march on Isengard. 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 Dwarves, 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 Dur 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 both fell in battle, they managed to defeat Sauron and bring him down. Isildur then subsequently cut the ring from Sauron's hand, thus 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, and the Orcs. Dwarves were also found on both sides, with those of Durin's folk, led by Durin IV, [approximately 20 - 50,000 Dwarves] fighting on the side of the Alliance.[Source?]

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.