Minas Ithil was taken by the Nazgul very soon after the Witch-King's utter defeat in Arnor by a Gondorian expeditionary force. That's right, a fraction of Gondor's forces can annihilate a great evil power which resides hundreds of miles away, but allow that same power to capture one of their most crucial fortresses, just a short ride from their capital city, with horrendous consequences for years to come. Go figure.
