When Aragorn became King of Gondor at the end of the War of the Ring, he was also the descendant of the Kings of Arnor and the Heir of Isildur, and by right he was crowned King of both Arnor and Gondor, later known as the King of the Reunited Kingdom and Reunited the Two Kingdoms, which had politically split long ago. Using the resources of the victorious Gondor, Aragorn, taking the royal name King Elessar, rebuilt the ancient ruined cities of Arnor in Eriador.
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 Lonely Mountain (Erebor) also known as Kingdom under the Mountain, although these allies maintained their own rule and kings. Aragorn was assumed to have rebuilt the old northern capital of Annúminas.