[Belegorn]

Originally Posted by tom the eldest

nah,there are civil war in arnor,but it was mainly rhudaur vs cardolan,arthedain still maintain peace,although maybe there are some difference with rhudaur.

I agree with cellurdur. It is said, "There was often strife between the kingdoms, which hastened the waning of the Dúnedain." [Appendix A., The North-kingdom and the Dúnedain] Remember, the Witch-king, "came north with the purpose of destroying the Dúnedain in Arnor, seeing hope in their disunion". They themselves aided in their own demise and their constant fighting gave the Witch-king hope that he might overcome them from Angmar in the north.

By the time of the 7th ruler of Arthedain there were no descendants of Isildur in the other 2 divisions of Arnor, although there were still a few Dúnedain still abiding in Rhudaur even though it was controlled by a lord of the Hillmen. In the 1600s a plague wiped out the Dúnedain of Cardolan and their last prince died in the war of 1409 with Angmar.

Originally Posted by Inziladun

it fell into decay, conquered by the Lord of the Nazgûl and his army of evil men. Why was that?

It was because the kingdom was not united.

Originally Posted by Inziladun

Their Tower of Amon Sûl was taken and razed.

They really dwindled in numbers quickly up north. About 200 years after this war the whole population of Dúnedain in Cardolan was wiped out by a plague. The Elves helped get them out of this jam.

Originally Posted by Inziladun

it has always seemed to me that the Witch-king's victory over them was easier than it should have been

Perhaps. I don't know. The Dúnedain had few children, had them late, or not at all, and were dying a lot. Gondor was the greater kingdom in any case. Consider as late of the 2nd to last king of Gondor, when he sent a small fleet to aid the last king of Arnor.

"Then Eärnil sent his son Eärnur north with a fleet, as swiftly as he could, and with as great strength as he could spare. Too late.... when Eärnur came to the Grey Havens there was joy and great wonder among both Elves and Men. So great in draught and so many were his ships that they could scarcely find harbourage, though both the Harlond and the Forlond were also filled; and from them descended an army of power, with munition and provision for a great war of kings. Or so it seemed to the people of the North, though this was but a SMALL sending-force of the whole might of Gondor." [Appendix A., Gondor and the heirs of Anárion]

This minor force seemed to the Men up north to be great. That's the difference between the two kingdoms and this is a Gondor in its decline.