Angmar war

Conflict with Angmar
Arnor's greatest enemy in the north by the middle of the Third Age was Angmar, ruled by the Witch King of Angmar. During the reign of Malvegil (c. T.A. 1300), this new power arose beyond the Ettenmoors. This land became populated with the Orcs and men of Sauron, and began attacking Rhudaur and Cardolan. Eventually this Witch-king was identified as in fact the chief of Sauron's Ringwraiths. Years later, Argeleb I of Arthedain, reasserted control over Cardolan, and fortified a line along the Weather Hills. Despite this action, Argeleb fell in battle with Angmar and its ally, Rhudaur. His son Arveleg, however, counterattacked in conjunction with Cardolan and drove the enemy back. He held this frontier in force for quite some years successfully.

Then in T.A. 1409, Angmar crossed the Hoarwell river into Cardolan and attacked again. This time, Weathertop was captured and Arveleg fell in battle. While Amon Sûl fell, the palantír was recovered and taken to Fornost. While Cardolan was laid waste, Angmar was eventually stopped by Elven forces from Lindon and Rivendell, where Elrond countered them.

The Great Plague
Another threat appeared to the northern successor kingdoms, this time one that swords and spears could not deter. A major plague began in the east, in the vicinity of the Sea of Rhûn, northeast of Mordor. After doing great damage in Rhovanion, it struck Osgiliath in Gondor in, killing King Telemnar and his family. In seriously affected areas, the plague killed 50% or more of the populace. This plague, which barely affected western Gondor, spread northwards to Cardolan.

In Cardolan, it struck severely, wiping out the last of Dúnedain of Cardolan at the Barrow-downs. The Witch-king, exploiting the tragedy, sent evil spirits, the Barrow-wights, to infest the area. The Hobbits of the Shire were damaged by it, but not heavily. The plague lost its strength, however, at this point, so that most of Arthedain was unaffected.

The Fall of Arthedain
In T.A. 1974, the final chapter in Arthedain's history began. The Witch-king attacked during the harsh winter weather. The capital of Fornost fell, and the remaining Arnorian forces were driven over the Lune river into Lindon. King Arvedui was compelled to flee to Forochel, and ask aid of the Snowmen there. His son, Aranarth, journeyed to Círdan at the Havens to inform him of Arthedain's fall. Círdan responded by sending a ship north to rescue Arvedui. When the Snowmen of Forochel saw the ship arrive, they were uncomfortable and nervous about the escape plan. Their chief replied to Arvedui: "Do not mount on this sea-monster! If they have them, let the seamen bring us food and other things that we need, and you may stay here till the Witch-king goes home. For in summer his power wanes; but now his breath is deadly, and his cold arm is long."

- The Lord of the Rings, Appendix A, p. 1018

It turned out that the Snowmen were right. A storm blew in that night and drove the ice towards the shore, and the ship was crushed and sank, with great loss of life, including King Arvedui. He unfortunately fulfilled Malbeth the Seer's prophecy about him at his birth that he would be the 'Last king' of Arthedain. So the North-kingdom ended, but the Hobbits survived in the Shire. They eventually chose a Thain from among themselves to replace the King, and the first of these was Bucca of the Marish, in T.A. 1979.

Showdown with Angmar
Envoys from Arthedain had journeyed to Gondor to ask assistance from the southern Dúnedain in fighting the Witch-king's forces. Gondor, however, was preoccupied with its own threats from the Easterlings, and so could not respond immediately. Gondor had been in a weakened condition since the death of King Ondoher and his two sons in the Battle of the Camp fighting the Easterlings in T.A. 1944. Arvedui of Arnor tried to claim the southern throne but this claim was rejected by Gondor. Eärnil, the victorious commander in the above battle and a member of the royal house, claimed the throne, and was confirmed by the Gondorian royal council.

King Arvedui sent increasingly urgent messages to Gondor about the crisis he faced from Angmar's continuing assaults. Eärnil II was unable to react quickly due to his need to order Gondor after succeeding to the throne, however. The king sent his son and heir Eärnur north to the Havens with a powerful fleet. Unfortunately, it was not in time to save Arthedain, and the northern kingdom perished. When Eärnur's naval forces landed in the Grey Havens, they dazzled both Men and Elves with their size and majesty. From these ships debarked the most powerful army seen in the north of Middle-earth in centuries. Círdan's people were quite impressed with the strength of Gondor's army, particularly its cavalry forces, dominated by riders from the Vales of Anduin. Círdan and Eärnur combined their forces, along with the remnant of Arnor's army, in the greatest joint Elf-Man army since the War of the Last Alliance; this great Host of the West re-crossed the River Lune and marched northward. These allies drove relentlessly toward the Arnorian capital of Fornost, where the Witch-king had occupied the palace complex there.

Angmar is Vanquished
When the Witch-king saw the invading Host, he failed to take it for the serious threat that it in fact was. Instead of awaiting the invaders in the fortress city of Fornost, he confidently marched his forces out to meet them in the open. He expected to defeat them as easily as had vanquished Arvedui's forces the previous year. But there was an appreciable difference this time-- the ground and naval might of Gondor. The allied Host continued to advance, and instead of establishing a merely defensive position, they attacked him from the Hills of Evendim, and a large battle broke out. The Witch-king's army could not stand before the allies, however, and began to retreat back towards their capital. Any hopes for an orderly withdrawl were in vain, however.

Gondorian cavalry forces, attacking from the north, routed the forces of Angmar, and put them to flight signaling an end to what became known as the Battle of Fornost. The Witch-king, in full flight, forsook his new conquests, and made for Angmar. But the cavalry under Eärnur continued the pursuit and rode down what remained of his forces. To add to his difficulties, an Elven force under Glorfindel also attacked from Rivendell, and completed his forces' destruction. At the last, the Witch-king charged Eärnur in frustration, but his horse shied away from the evil wraith. As Eärnur once again mastered his horse, Glorfindel uttered his famous prophecy: "Do not pursue him! He will not return to this land. Far off yet is his doom, and not by the hand of man will he fall."

- The Lord of the Rings, Appendix A, p. 1027.

This prophecy would not be fulfilled until a thousand years later, at the Battle of the Pelennor Fields.