[Inziladun]

Originally Posted by Moonraker

The real battle had not started yet when Gandalf confronted the Witch King. Also, the hosts of Mordor would have been mortified with fear and amazement at seeing their so-called invincible leader falling to a superior power before their eyes. It would have mesmerised them. The Witch King was slain in the midst of battle where far fewer soldiers would have seen his fall, and there was no time to pause for reflection.

Sauron's forces were not tied to a specific field commander, even one as singular as the Witch-king. It is said more than once in the books that it was Sauron's will that drove his armies forward against all doubt and fear. Even if thousands of them had seen the WK's fall, they would not have abandoned the battle.

Originally Posted by Moonraker

Would anyone else have had the guts to take on Gandalf the White? The Uruk Hai of Isengard fled with terror at his coming. One thing to consider is whether Gandalf would have broken the rules bestowed on the Istari to not reveal their true power against Sauron directly. In slaying the Witch King, he probably would have had to have used is true Maia powers, thus dying himself as a consequence. But that is one possibility, and Gandalf living on to fight another day to continue the battle against Sauron is another.

Gandalf revealing himself "uncloaked" as the White was reserved for very special occasions, such as when he rode to rescue Faramir. It was only allowed when no other option existed. It is notable that at the confrontation at the Gate with the Witch-king he does not display any overt power. I believe Gandalf, possessing a self-acknowledged amount of foresight, could have had an idea that the WK was to soon meet his fate, though lacking the details. At any rate, Gandalf did not take to the field of battle to confront him; he only wanted to prevent him from leading his host in a sack of Minas Tirith. Gandalf had the power to destroy him, but in keeping with the limitations he worked under, still left actual fighting to others.

Originally Posted by Aiwendil

That's debatable, since the prophecy, if I recall correctly, said rather than no mortal man would kill the Witch-king. Does an incarnated Istar count as a mortal man? Though Tolkien's solution of having him killed by a mortal woman is, of course, nicer.

The quote from Glorfindel in Appendix A only says "not by the hand of man will he fall".