The Union of Númenor began shortly after the coronation of Skelesam as 2nd Chieftan of Arnor, and Atanvarno 5th King of Gondor. Shortly after the Union began, the Shire under Thain_Vibiras also joined, adding further to the strength of the Union. Following the conquest of Umbar, they too joined the Union.
The aim of this Union is to unite the Dúnedain of the North and South to create an even larger force to work for the Good of Middle Earth, and defend it from those who would see the Free People harmed.
The ancient kingdom of Gondor was one of the two founding factions in the Union Of Númenor. Comprising the Dúnedain of the South, Gondor is constantly on alert for the Shadow in the East.
The other founding faction was Arnor, more commonly known as the Rangers of the North. The shattered kingdom of Arnor is made up of the Dúnedain of the North and have long watched over the lands of Eriador and remained vigilant against the hordes of Gundabad and Angmar.
The Little Folk of the Shire, the Hobbits. The Shire was the 3rd faction to join the Union. Relations between the Shire and the Dúnedain have long been friendly so the Shire was quick to be welcomed in and became instrumental in Númenor's decisions.
Following the War Of Reconquest, Umbar surrendered and joined Númenor. The capital was being built by Kvesir and another city was being built by Thain_Vibiras. Following the expulsion of Kvesir and his Corsairs after they were found to be traitors Númenor, Thains city became the capitol of Umbar.
The end of the Union came when the Arnorian Chieftain Elendil_The_Tall withdrew from the Union. Following Arnor's withdrawal, Numenorian Umbar was made directly part of Gondor, and the alliance became between only Gondor and the Hobbits. While this lasted for a time, the two parties finally decided to end the declining Union, and it at last was disbanded.