Last Alliance of Elves and Men
Sauron had been contending with the Elves for mastery of Middle-earth for over 1200 years. Fearing the establishment of the powerful kingdoms in exile — Arnor and Gondor — by his hated enemies, the Dúnedain, Sauron launched a preemptive attack on Gondor in S.A. 3429.
In response, Elendil, High King of the Dúnedain, formed an alliance with Gil-galad, the High King of the Eldar, to repel Sauron's assault. Durin's folk, under command of King Durin IV, also aided Elendil and Gil-galad.
The hosts of Gil-galad and Elendil met at Amon Sûl and marched towards Imladris where they camped for three years, forging armoury and making plans whilst Anarion, Elendil's younger son, defended Osgiliath agains the hosts of Mordor.
The armies crossed the Misty Mountains at various locations and there forces were strengthened in the vale of Anduin by Elves from Lothlórien and Greenwood the Great under the command of Oropher and Amdír and by the Dwarves of Khazad-dûm.
In S.A. 3434 they defeated Sauron's army in the Battle of Dagorlad, breeched the Morannon into Mordor, and besieged the Dark Lord's fortress of Barad-dûr. The siege lasted for seven years, during which Anárion was slain. It culminated in Sauron leaving his fortress and engaging in direct combat. There were three objectives to this war; to unmake the One Ring, to destroy Sauron, and to remove the foundation of the Dark Tower.
Sauron was felled by Gil-galad and Elendil, who both perished in the assault themselves. Elendil broke his sword Narsil as he fell. Using the hilt-shard of the sword, Isildur cut the Ring from Sauron's finger. Bereft of the power of the One Ring, Sauron's spirit dissipated and would not take form again in Middle-earth for a thousand years.
With the victory over Sauron, and the death of Gil-galad and Elendil, the Last Alliance was dissolved. Durin IV survived the war and afterwards returned to Khazad-dûm, and the remaining Númenóreans resettled their kingdoms.