Arnor was once a great kingdom of Men in the north of Middle-earth, between the Hithaeglir (the Misty Mountains) in the east, and the Ered Luin (Blue Mountains) on the borders of Lindon in the west, and encompassed the area where the Shire would be founded many years later.
Division into Arthedain, Cardolan, and Rhudaur
Eärendur was the tenth king of Arnor. When he died in T.A. 861, his sons divided his kingdom between them. Amlaith, the eldest son (who according to tradition had the proper claim on the whole of Arnor and was the Heir of Isildur) took as his part the region known as Arthedain, which included the Shire, much of Bree-land and the North Downs, and the tower of Amon Sûl (Weathertop). Amlaith's two brothers founded the realms of Cardolan and Rhudaur. The old capital of Annúminas fell into decay and within a few centuries was nothing but a ruin. Amlaith moved the capital to Fornost, which flourished for roughly a thousand years.
At around T.A. 1300, the kingdom of Angmar arose to the North-east of Arthedain, ruled by a terrible and fierce lord, styled the Witch-king of Angmar. It was later discovered that this was the leader of the Nazgûl, and that this was all part of Sauron's plan to control all of Middle-earth.
Over time, Rhudaur became home to more and more hill-men from the North. In T.A. 1356, hill-men from Angmar assaulted Arthedain and Rhudaur, resulting in the death of Argeleb I, the seventh King of Arthedain. The hill-men took control of Rhudaur from that point, and the kingdom soon became a vassal of Angmar.
Soon after, in T.A. 1409, Angmar invaded again. This time king Arveleg I of Arthedain was slain, and the great tower at Amon Sûl was razed. The Dúnedain were able to rescue the great palantír from the invaders though.
In T.A. 1974 the Witch-king of Angmar invaded Arthedain and took the city of Fornost, occupying it and sacking it thoroughly. In T.A. 1975, a coalition of forces from Gondor led by Eãrnur, along with forces from Rivendell led by Glorfindel, drove the forces of Angmar out of Arthedain, but at a heavy cost. Most of the population of Arthedain was slain and dispersed. The remnants of the Dúnedain took to calling themselves the Rangers of the North after this point, and became a secretive folk, protecting their kingdom of old.
The Beacon of Arnor