LotR -- Book 3 - Chapter 05 - The White Rider

[Tuor of Gondolin]

One of the aspects about LOTR which I like is the occasional "history
and/or background" observations/commentaries. Gandalf's here are
especially revealing, especially explaining what would otherwise seem
rather curious, Sauron's frantic haste in attacking Minas Tirith and his
overall psychological makeup.

...he does not yet perceive our purpose clearly. He supposes that we were all going to Minas Tirith; for that is what he would himself have done in our place...Indeed he is in great fear, not knowing what mighty one may suddenly appear, wielding the Ring, and assailing him with war...Wise fool. For if he had used all his power to guard Mordor, so that none could enter, and bent all his guile to the hunting of the Ring, then indeed all hope would have faded: neither Ring nor bearer could long have eluded him.

Also, the above observation of "not knowing what mighty one" seems at variance with the general view, shared by Tolkien himself in "Letters", that
probably only Gandalf could use the Ring against him (ignore PJ's movie
view in toto about no one using the Ring, that's an overinterpretaion/simplistic
view). Gandalf's statement here, which seems to be quite definitive (and by Gandalf the White) would seem to say that a number of candidates might use the Ring: Denethor, Theoden, Gandalf, Aragorn, Faramir, etc. And how would the Ring react to a woman wielding it (Eowyn)? Perhaps that would really put Sauron
off his game.