During the reign of Eärendur his sons were in open discord, which erupted into civil war after Eärendur's death in Third Age 861. Amlaith, the true heir to the throne of Arnor, was opposed by his two brothers. Unable to resolve the situation, he was reduced to ruling the region of Arthedain, which consisted approximately of the lands west of Baranduin River and north of the Great East Road. (His brothers created the kingdoms of Cardolan and Rhudaur to the south.)
In 1300 the kingdom of Angmar appeared at Arthedain's north-eastern border. Its King was the chief of the Ringwraiths, although this was not known to the Dúnedain. When this new threat came Cardolan placed itself under the suzerainty of Arthedain, which then began to call itself Arnor again. Cardolan repeatedly sent aid to Arthedain when needed but by 1409 Cardolan and Rhudaur were conquered by Angmar.
Arthedain kept up the resistance against Angmar for over 500 years but in 1974 Arthedain was overrun and Fornost fell into the hands of Angmar. It fell just one year too soon for help was underway from Gondor under the lead of Eärnur. This army reached Arthedain in 1975 and destroyed Angmar at the Battle of Fornost.