This meanders a bit at the start, but persevere and it'll make sense...

Ok - In T.A.1944 the Wainriders attacked Gondor ( at the same time as Harad did in the south). Gondor split it's army into two.. The Northern one being the larger. An estimate for it's size? I put it at Approx. 12,000 for the North (as it says that it was the larger - that's Cirion and Eorl, Unfinished Tales)and (,000 for the Southern Army.
Why? Well, in the Last Debate (LOTR), Prince Imrahil Says:
"We should ride with seven thousands, scarce as many as the Vanguard of it's army in the days of it's power"

Most army were divided into three 'Battles', with the vanguard forming the lead. The vanguard was usually the smallest part as well, so a figure of 21,000 troops for Gondors field army may be a bit conservative. Don't forget this is still at the time of the Kings, when the realm was still large.
Right - In the battle of the Wainriders, the King ( can't remember his name! Didn't write it down!) is killed along with part of the centre of the Army.The left and right flanks were retreated ( I don't think they had had time to come into contact with the enemy) and formed up in Ithilien behind a regard, which collapsed eventually and went to find the remainder of the Army. assuming that the Army was divided roughly equally into three, this puts the losses at a maximum of 4,000 troops, i.e the whole of the centre.
The southern Army we are told won a great victory in the south and then came and joined the rest of gondors army ( presumably the northern part) and won the battle of the camp, routing the Wainriders.
Assuming that losses were not as heavy for the southern army, as they were victorious, say approx 1000-2000 men, this puts Gondors field army at the end of 1944 at roughly 16,000 men, at the least.Don't forget Minas Tirith, the Undeeps, The Hornburg, Orthanc were all still manned, as well as Minas Ithil, Pelagir etc..All this is from Cirion and Eorl,Unfinished Tales, by the way.
So... 30 yrs later, time enough for new people to be born, losses to be made good etc, the end of the North Kingdom in 1975.
Earnill 2 sends an army to the North...
"An army of power, with munitions and provision for a war of great 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" - From Appendix A.
This implies large military forces,and makes my figure of a 16,000 strong standing army seem inadequate. But there's more...
25 yrs later, the Nazgul besiege Minas Ithil. 25 yrs of relative peace for Gondor, Time enough to recover from the war in the North.
So gondor, with a minimum army of 16,000, at the minimum, led by a General with good reason to fight the Witch King ( as His horse bore him away in the battle for the North Kingdom), couldn't find itself able to raise a siege from an important fortress where the Palantir, a vital communicating tool, is kept?
People have previously said that despair would have been used, but surely the Palantir could have been used to keep the defenders abreast of the situation? Show them that a relief force was on the way?
Since there was a palantir there, would not a large Garrison be present to defend it? Minas Ithil is descibed as a Fortress, which puts it in the same vain as Orthanc,Hornburg, even Minas Tirith.
So the question remains...
How the Hell did the Nazgul defeat Gondors entire army ( of at LEAST 16,000,and very likely a lot more) and take as strong a citadel as Minas Ithil?

WELL???????

:)