A proto-Angmar in the Second Age

[William Cloud Hicklin]

I rather think that Sauron was desperately trying to regroup after his surprise dis-embodiment in Numenor, and scraping up enough troops just to deal with Gondor. And the notion of offensive ops against Elendil and pre-Last Alliance Gil-Galad/Lindon strikes me as Sauron sticking his head in a hornet's nest. One Nazgul? Curbstomp (let's not forget that the W-K was afraid of one Noldo, Glorfindel. You think he can take an army of them?)

The 3rd Age Angmar project took centuries to pay dividends- against a weak and divided Arnor.

[Zigûr]

Yes. While referring to Lindon, not Arnor, Appendix A is entirely explicit on this point:

But Sauron struck too soon, before his own power was rebuilt, whereas the power of Gil-galad had increased in his absence; and in the Last Alliance that was made against him Sauron was overthrown and the One Ring was taken from him. So ended the Second Age.

It's emblematic of how doubtful Sauron's means were at the end of the Second Age that his enemies were able to invade his territory, rather than the reverse. Some short-lived early victories against newly-founded Gondor notwithstanding, Sauron was weak* at the end of the Second Age, and his enemies were strong. That's why he spent the next three thousand years wearing his enemies down through subterfuge, unconventional weaponry and proxy wars.
*And this is taking into account the remark in Myths Transformed about Morgoth's First Age power compared to Sauron's in the Second.

[William Cloud Hicklin]

It's also the case that Tolkien said (can't find the source at the moment) that a significant factor in Arnor's later weakness was that it was stunted in childhood, by the massacre of most of its fighting men at the Gladden Fields.