How did the Witch-king take and hold Minas Ithil so easily?

Question

One of the longer conflicts in the Legendarium is the Fall of Arnor, where the Witch-king spends a century building up Angmar to rival the diminished and disunited petty kingdoms of the Dúnedain, followed by nearly six hundred years of fighting before Arnor is destroyed.

c. 1300 The Nazgûl reappear. The chief of these comes north to Angmar
1409 The Witch-king of Angmar invades Arnor
1974 End of the North-kingdom. The Witch King overruns Arthedain and takes Fornost.
_{The Return of the King, LoTR Appendix B, The Third Age}

This war concludes with Gondor shipping what it considers a small expedition across the continent and utterly vanquishing recently victorious Witch-king's realm:

But 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...

But the Host of the West came down on him out of the Hills of Evendim, and there was a great battle on the plain between Nenuial and the North Downs. The forces of Angmar were already giving way and retreating towards Fornost when the main body of the horsemen that had passed round the hills came down from the north and scattered them in a great rout. Then the Witch-king, with all that he could gather from the wreck, fled northwards, seeking his own land of Angmar. Before he could gain the shelter of Carn Dûm the cavalry of Gondor overtook him with Eärnur riding at their head. At the same time a force under Glorfindel the Elf-lord came up out of Rivendell. Then so utterly was Angmar defeated that not a man nor an orc of that realm remained west of the Mountains.
_{The Return of the King, LoTR Appendix A, Annals of the Kings and Rulers: The Realms in Exile: The Southern Line: Heirs of Anárion}

Then comes something I've never quite understood: the defeated Witch-king packs up and heads to the doorstep of the same powerhouse that just annihilated his seven centuries of empire-making and proceeds to take one of its fortified cities in short order:

1980 The Witch King comes to Mordor and there gathers the Nazgûl.
2000 The Nazgûl issue from Mordor and besiege Minas Ithil.
2002 Fall of Minas Ithil, afterwards known as Minas Morgul
2043 Eärnur becomes King of Gondor. He is challenged by the the Witch-king.
2050 The challenge is renewed. Eärnur rides to Minas Morgul and is lost. Mardil becomes the first Ruling Steward.
2063 ...The Nazgûl remain quiet in Minas Morgul.
_{The Return of the King, LoTR Appendix B, The Third Age}

Aside from resisting the initial siege for two years, Gondor isn't recorded as opposing this new power, even as the Witch-king taunts and almost certainly murders their king. In fact, Minas Morgul remains a hostile power for more than a thousand years, until Sauron's final defeat.

So it seems that either the Witch-king can muster a vastly more powerful force over the course of 20 years in Mordor (which is to say, a wasteland that has been watched by Gondor until fairly recently) than he can over centuries in Angmar, and/or that Gondor is not very interested in the retaking of one of their most prestigious and strategic city-fortresses, even as this fortress causes problems like dead rulers and ravaging Orc armies over the centuries.

Is this situation ever explained?

Answer 1

Gondor was already past its prime at this point, having abandoned Osgiliath for Minas Anor/Tirith. The comparison to Arnor is strange as well. You are comparing a kingdom to a single city, a city that was intentionally built on the edge of Mordor already, and at this point not very largely populated.

TA 1640 King Tarondor removes the King's House to Minas Anor, plants a seedling of the White Tree. Osgiliath begins to fall into ruin. Mordor is left unguarded.

That and, as far as I know, we don't know the size or make up of the force that attacked Minas Ithil. Angmar was fairly isolated, no allies remotely close, other than perhaps Orcs in the Misty Mountains. Once in Mordor, the Witch-king could have potentially drawn from the east and south from peoples who had already contributed to Gondor's decline.

Answer 2

The People of the North had lower population numbers than the people of other places, by the text (small force was seen as impossibly large). Mordor, for all being a wasteland, seems at least to have the capacity to feed and house large numbers of Orcs and Goblins in later eras. Potentially there were local tribes or smaller states that simply lacked the will to take on the Gondorians, but had the numbers. There're a lot of historical examples of this kind of thing - deposed princeling, whose family has ruled a rich area for several hundred years gets kicked out, finds allies in tribes in some horrifyingly inhospitable area worth about a millionth of the value of the rich area, comes back and burns several Persian cities, and they give him a satrapy so he stops burning things.

Military might and length of time spent ruling something are not necessarily correlated.

Likewise, Orcs can apparently grow in numbers very very swiftly if there is need. Perhaps the Witch-king induced that sort of massive military build-up to take on Arnor. Maybe without the Nazgûl he didn't feel up to it, or he was still feeling that ancient body blow. So over a few years he creates a huge army, takes down Arnor, and loses most of the army doing it (which is intentional - he doesn't want to feed them). Then the Gondorians show up, his pants are down, his army is gone, and he doesn't have time to make a new one. They slash and burn and knock down all his stuff; what armed forces he has panic and are worse than useless; he heads off to Mordor, finds some local Orcs, creates a new army and takes Minas Ithil with it. It's barely defended; the Gondorians haven't been watching Mordor for years.

Forces that are useful under some circumstances, but which fall apart when confronted with a feared foe (like heavy cavalry), is also extremely historically realistic. There're armies that took cities and beat other armies and then lost to certain opponents due to lack of officers, organization, supplies or tactics with casualty rates of hundreds to one: total routs where one side lost basically nothing and the other was utterly destroyed.

Then he kills off Eärnur by effectively tricking him into a duel. Death by pride. Stewards take over lesser men who are more easily manipulated. The Witch-king always prefers manipulation, so he puts that down as a slow burn project and focuses on other more arcane projects like recovering his mojo, corrupting fate or killing off the Wizards and the Elves or whatever.

These are the two main explanations that I can see:

1. The Witch-king musters an army in Mordor which is a serious challenge to Gondorian power. This is exactly how historical armies worked - you mustered them, they weren't permanent standing armies, and a powerful state could be defeated by an unexpected attack that did not allow them to muster (such as the conquest of the Khwarazmian Empire by Genghis Khan).

2. The Witch-king manipulates Gondor after killing off the man with the vision to see what a threat he is, leading to a slowly weakening and poorly led Gondor sliding further into obscurity to be defeated later on. This fits perfectly with later events that we see in the books.

It could even be both: A manipulation based on the creation and mustering of a serious army out of the Mordor badlands and local Orkish tribes that Gondor can be convinced it does not want to take on.