[Boromir88]

I think Legate's guess is as good as any. We aren't really told of any details (besides what Legate has quoted in Appendix A - which is all that's said in my published version)...The Witch-King gathered the other Nazgul in Mordor and laid siege to Minas Ithil.

If it was a siege for two years we'd assume there was an army involved as well...as I agree it doesn't make sense that 9 Nazgul would be outside the gates for two years.

As Legate also mentions Boromir credits the Witch-King with being the most influential in Gondor losing the western half of Osgiliath:

...;but it was not by numbers that we were defeated. A power was there that we have not felt before.~The Council of Elrond

And that power was as Boromir would call...'The Black Captain.'

There's not really much more to add, except I don't think this 'siege' would be comparable to that of the siege of Minas Tirith. Eventhough if it did last longer, there probably wasn't a great amount of forces involved. A description of Mordor during King Hyarmendacil's reign in T.A. 1050 is this:

"Mordor was desolate, but was watched over by great fortresses that guarded the passes."~Appendix A: Gondor and the Heir's of Anarion

This would be referring to Minas Ithil, then it falls in T.A. 2002.

I don't think it was a huge battle at all, there was probably relatively low amount of forces involved. As after the fall of Minas Ithil, it is pretty much ignored. Nobody suspects anything, the Council of the Wise don't think Sauron has come back until over 50 years later (T.A. 2060)...that is when the Council of the Wise begin to start watching out for Sauron more, as the siege upon Minas Ithil seems to be rather passed off as 'unimportant.' And people don't start worrying about Sauron until half a century later.

After Sauron returns Mordor's forces start growing exponentially and assaults upon Gondor start renewing. My guess is that after Sauron's fall, there was some watch put around Mordor to see if Sauron would come back. But after some many years of 'inactivity' and Mordor being 'desolate,' plus with assaults from Angmar and Dol Guldur, that seemed to grab attention more than out in the 'desolate' Mordor. So, watching Mordor became less important as to watching Angmar and Dol Guldur.

That's just when the Witch-King slips away out of the North, gathers the Nazgul and launches an assault on Minas Ithil, taking it 2 years later. Even after that, the Council of the Wise and others still don't think much of it it seems.