Angmar was founded in 1300 T.A. by the Lord of the Ringwraiths who ruled it as the "Witch-king". It was therefore a vassal to Sauron
Soon after it was founded it started dangerous attacks on the fractured remains of the Dúnedain kingdom of Arnor (divided into Arthedain, Cardolan and Rhudaur)
The Witch-king took over Rhudaur, the weakest of Arnor's remains by supplanting its Dúnedain king by a native Hillman.
In 1356 T.A. Rhudaur invaded Arthedain on Angmar's bidding and its King Argeleb I was slain but with the help of Cardolan Arthedain managed to hold the defence perimeter at the Weather Hills.
In 1409 T.A. Cardolan was destroyed and Rhudaur was finally terminated by the Witch-king.
Arthedain was left without allies but struggled on for another 500 years. In 1974 T.A. Angmar amassed its forces and launched its final assault on Arthedain. They took its capital Fornost and thereby destroyed the last remainder of Arnor.
In the next year Prince Eärnur of Gondor arrived to save Arthedain, although he was too late he defeated the Witch-king in the Battle of Fornost and destroyed Angmar.
The Eótheód from which the Rohirrim descended moved as soon as they heard about Angmar's fall to get more lands to live on.