The Fourth Arnor (Arastil & Vardamir)
It was founded at the start of the Fifth Age, it's founder, Arastil Starfall, a very talented architect and good leader was not of Royal Blood and was theoretically a Regent or a Steward however one may want to word it. His reign lasted 24 years and mostly consisted of building and re-building, sowing the seeds for the years to come. Later on the kingdom fell into the hands of Vardamir Elrossirion who allocated a lot of resources in expanding the lands of the kingdom.
After the days of Bragolmor, the people of Arnor slightly diminished. The kingdom reduced to size. Only a handful of Earls and a single Lord survived. A Dunedain of the Starfall family, a lesser branch family of the Lords of Andúnië who had been hiding in Southern Ered Luin for decades until the heads of several Dunedain Families elected Arastil Starfall as king, granting him some sort of legitimacy over the Kingdom of Arnor. He was crowned King, Tar-Arastil by the Emperor of the Northern Pact in year 138 VA.
A New Figure
Twenty or so years later the crown's place did seem instable. Arastil got sick and was not very keen on one of his family members as ruler, since he had no child to groom to be a ruler. A Dunedain called Vardamir befriended Arastil, and without a doubt enlightened him. As the circumstances were Vardamir persuaded Arastil to sign a deed of relinquishment and to nominate him as the rightful ruler. Besides that Vardamir was of the Elrossirion family which is the Royal Family of Arnor.
Year 162 of the Fifth Age, marks the year Arastil officially abdicated and the year Vardamir got coronated. It also marks the year of great reforms within Empire.
Years 162, 163 and 164 of the Fifth Age, mark a great effort to expand the lands of Arnor. The expansion consisted the colonization of the lands bordering the river Gwathló, most of Minhiriath, and a small portion of land North of Annuminas.
The Third Arnor (Surion)
In FO 420, after 150 years of the fall of the 2nd Arnor, the Empire of the North was reunited once more under the reign of High King Surion Elrossirion.
The Second Arnor (Halbarad)
In FO 266, the proud Kingdom of Arnor has risen under the rule of High King Halbarad, the Empire grown mighty and desired all their former borders restored. The Dunedain of the North risen up and proclaimed their mighty and proud ancestry as the Sons of Numenor, and let all of Middle-Earth know their power.
War between Arnor and Angmar
The Shadow of Angmar has struck, and defeated the forces of Arnor in the North Downs. An army composed by Arnor and some free folks allies fought a last stand in the Northshire point Fort. Angmar (with some reinforcements from Rhudel and other evil factions) won without losses the pitched battle and then sieged the Fort. The evil side won the war, Northshire point became Angmar and the province of North Downs passed under the control of the Shadow.
The Casmir Coronation
A new king was crowned in a ritual by High King Halbarad, who believed he was unworthy of the Crown. The New ruler was High King Casmir I Engoria. With his rule Gondor has assailed the noble men of Arnor, which with wisdom and planning had a chance of destroying the forces of Gondor, who seek to destroy his Kingdom.
Casmir to his loyal soldiers:
"Tôl Acharn!" (“vengeance comes”)
War against Gondor
Under the reign of High King Casmir I Engoria, the Arnor empire was attacked by the Gondorian Empire. Emperor Amandil declared war to Arnor using "Imperial Conflict" as casus belli. Before the battle of Tharbad Plains, Gondor made some heavy raids into the Arnor villages. The two armies fought in a pitched battle and gondor won. In the last battle of the Gondor - Arnor war, most of the casmir side left his high king and just few men fought against the gondorian side.
Gondor won the war, annexing the contested province and occuping Tharbad. Arnor was dissolved after the end of the conflict. Riverlands, which was part of Arnor, joined Gondor after their former Empire fall.