[Belegorn]

Originally Posted by Inziladun

That supposition would seem to be borne out by the fact that they ended before Arthedain.

Perhaps, but I think Arthedain had the comfort of allies [the Elves] to aid in its endurance. During WW2 there were two powers the Allies were worried about, Germany and Japan. They felt Germany was the predominating power and therefore sought to oust them first of all. I'm not necessarily saying that Cardolan and Rhudaur were stronger because they went out first. I'm suggesting the possiblility that their fall before Arthedain does not necessarily imply they were at first lesser states. It would appear that eventually they did become lesser states to Arthedain probably due to the diminishment of their kings and princes and eventually the failure of those noble lines. Also Arthedain had the friendship of Rivendell and especially of Círdan and Lindon.

How could the kingdom have divided? Was Eärendur's heir not really fit to be king that he let the realm be divided? In Gondor during the Kin-strife a cousin of the rightful king, Castamir, with the backing of other nobles was able to usurp power. Ten years later Eldacar got it back.

1437 Burning of Osgiliath and loss of the palantír. Eldacar flees to Rhovanian; his son Ornendil is murdered.

1447 Eldacar returns and drives out the usurper Castamir. Battle of the Crossings of Erui. Siege of Pelargir.

1448 Rebels escape and seize Umbar.

In the North, however, the authority of the king was also challenged, but in this case the lands were "divided into petty realms and lordships" [Sil; Of the Rings of Power and the Third Age] It is said that Arthedain claimed the overlordship but it was disputed. So maybe the two younger brothers seceded rather than being given their own states?

Originally Posted by Appendix A: The North-kingdom and the Dúnedain

Both Rhudaur and Cardolan desired to possess Amon Sûl (Weathertop)... for the Tower of Amon Sûl held the chief Palantír of the North

This palantír was mainly used to commune with Gondor. Did these other two brothers have some sort of relationship with Gondor or maybe they wanted their own palantír to keep an eye on the movements of Arthedain which held the Annúminas-stone?

In The Heirs of Elendil it's said, "The lesser realms resist the claim of the king at Fornost to be overlord of all the former lands of Arnor." This is during the reign of Malvegil when the royalty in the other two kingdoms was no more which may be why they have now been called "lesser realms". Also it is said that in Rhudaur "power had been seized by an evil lord of the Hillmen, who was in secret league with Angmar." This realm at least had not only lost its kings but was also under the sway of Angmar. Not quite the realm it once was, though it was petty before.

I think that Arthedain may have survived longer because not only did they receive held from the Men of Cardolan when Angmar tried to take Weathertop, they also still had friendship with the Elves who helped them.

Originally Posted by PoM-e, ch. 7, The Heirs of Elendil

the Dúnedain of Arthedain repel the forces of Angmar from Fornost with the help of Cirdan of Lindon.

Originally Posted by PoM-e, ch. 7, The Heirs of Elendil

Amlaith and his descendants maintained friendship with the Eldar, especially with Cirdan at the Havens.

Originally Posted by Appendix A: The North-kingdom and the Dúnedain

Arveleg son of Argeleb, with the help of Cardolan and Lindon, drove back his enemies from the Hills

Originally Posted by Appendix A: The North-kingdom and the Dúnedain

Araphor son of Arveleg was not yet full-grown, but he was valiant, and with aid from Círdan he repelled the enemy from Fornost and the North Downs.

Perhaps Angmar was finally able to sink its teeth into Rhudaur when its king died and was thus able to get men on the inside. This probably did not happen in Cardolan because they still had their princes until Angmar came out in force in 1409. Sometime in the late 1200s "evil came to Arnor", in what form I do not know. "There was often strife between the kingdoms" so I'm not sure in what guise this evil came. I do not think it was open war. Perhaps there were some skirmishes, and eventually the internal power grab in Rhudaur.

I do not know if Arthedain is ever pointed out to be the strongest of the three realms. Did the other two "lesser realms" fight together against Arthedain or were they all fighting each other for all those centuries? Maybe there was some teaming up here and there, most likely with Rhudaur and Cardolan, just as Cardolan joined forces with Arthedain against Rhudaur, although in this case it was more like a joint venture against Angmar since there were insiders in Rhudaur who eventually expelled the Dúnedain from those lands.

Just going through this stuff I'm probably going more along the lines of either the realms were near-equals although Arthedain had allies in the Elves, or the other two were "lesser realms" who teamed up. But why not just become one state? If they were not good with their older brother being king, were they also not good with each other being king?