[Child of the 7th Age]

Aldarion,

Excuse me for popping in again. I am thinking in shorthand and not explaining things in detail.

Good question that needs more explanation! What I meant to say was that Gandalf could not win the fight except by giving up his own life. There are hints of this in his earlier reluctance to go through the mines of Moria. He sensed that something waited for him there that, by himself, he could not overpower and simply walk away.

By agreeing to give up his own life, Gandalf was seemingly letting go of all chances of personally defeating Sauron. This is something that Tolkien himself discusses in the Letters. I don't have them at hand, but I recall that he said something to the effect that Gandalf agreed to lay his own interests aside and hand matters over to the "Authority" (meaning Eru) trusting that things would somehow come out right in the end.

Hope this helps.