Aragorn was the descendant of the Kings of Arnor and the Heir of Isildur, and thus, when he became King of Gondor at the end of the War of the Ring, he was crowned by right king of both Arnor and Gondor. Using the resources of the victorious Gondor, Aragorn took the royal name King Elessar, reunited the Two Kingdoms (which had politically split long ago), and rebuilt the ancient ruined cities of Arnor in Eriador. With the defeat of Sauron and Mordor, the Reunited Kingdom became the dominant power of northwest Middle-earth, retaking all territories that originally belonged to both Arnor and Gondor at their greatest extents, excluding only Rohan and the Shire.
Additionally, during the first century of the Fourth Age Aragorn led the armies of the Reunited Kingdom in many successful military campaigns that:
- retook the province of South Gondor (Harondor)
- re-established dominance over Rhovanion (Wilderland) eastwards to the Sea of Rhûn
- conquered the Corsairs of Umbar and the Southrons, who had attacked Gondor during the War of the Ring and made Umbar a province of Gondor once again.
Umbar had originally been a Númenórean port and an important city of Gondor in ancient times before ultimately being conquered by the followers of Castamir following the Kin-strife, who distanced Umbar from Gondor and befriended Harad. Elessar finally reconquered the city, and the Reunited Kingdom stretched south along the coast from the Mouths of the Anduin River to the Haven of Umbar. Much of Harad was defeated and became tributary states. Aragorn was also held to be overlord of Dale (which included Laketown (Esgaroth)), and the Dwarf-city of the Lonely Mountain (Erebor) also known as Kingdom under the Mountain, although these allies maintained their own rule and kings. Aragorn is assumed to have rebuilt the old northern capital of Annúminas as well.