The Northern Kingdoms

[Inziladun]

Originally Posted by William Cloud Hicklin

Gondor did- it is IMO pretty clear that Harad (under Sauron/Nazgul influence, and its ruling caste of Black Numenoreans) were backing Castamir in fomenting the Kinstrife.

Appendix A states Castamir was "supported by the people of the coasts and of the great havens of Pelargir and Umbar".
After Castamir's defeat his sons sought refuge at Umbar, and then it seems they made their alliance with "enemies of Gondor".

Originally Posted by William Cloud Hicklin

Arnor broke up long before the Witch-King came north- in fact it was the rise of Angmar which forced the squabbling sub-kingdoms to unite (or at least Arthedain and Cardolan, Rhudaur being a lost cause)

Certainly Arnor was already fractured at the WK's arrival , but it looks like he really hurt any chances of a unified Arnor when his vassals took power in Rhudaur. The Dúnedain were overwhelmed by superior numbers, it seems.