The Reunited Kingdom was a Kingdom founded by Aragorn II Elessar after the War of the Ring. The main two Kingdoms that were united was Gondor and Arnor.
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, refurbished and reunited the Two Kingdoms (of which Arnor had politically split and ceased to be 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.
Aragorn also led military campaigns, thus adding Harandor (South Gondor), Rhovanion (Wilderland), Harad and Umbar. He was the first Gondorian in many years to realize there was an organization by the name of Heavenstone. At first King Ellesar did not bother with getting rid of it. 15 months after the invasion of Umbar, Heavenstone launched an expedition, and succesfully found Númenor, which was discovered by the high summit of Mentelarma. After Heavenstone was testing their weapons on elves and other men, King Ellesar openly declared to shut down the whole Heavenstone organization. Heavenstone refused.
"We long have been independent from Gondor's rule. When we secretly aided them, they forgot us. As a result, we aren't in their control. If King Ellesar thinks that he has rights over Heavenstone, the organization that keeps you safe, he is wrong. We were once part of Isildur's Gondor. And we fought for it, died for it. But soon they forgot about us, slowly, slowly, slower than any dwarf. And we did too. We have no connection to the Kings of Gondor, as they had no connection to us".
---Commander of Heavenstone's Army and Navy, Ingold III
After this, there is no or very little information of the connection of Heavenstone and King Ellesar.
The military of the Reunited Kingdom was mostly the same as Arnorian and Gondorian troops as they were in the 2nd Age of the Sun. Since Heavenstone left Gondor, the military slowly worsened a little.
By far the Reunited Kingdom has the most land in all of Middle Earth history. The following are the subjects of this Kingdom:
- South Gondor/Harandor
- Ithilien Wasteland and Ithilien
- Near Harad
- Small settlements in Far Harad where the Stars are Strange
- Small ruins of Angmar
- Small ruins of Mordor
- Many of the Vales of Anduin (leaving the Northern Half for the Beornings)
- Númenor (peak of Mentelarma)
- Morgul Vale