[Tuor in Gondolin]

Originally posted by Kuruharan
I've had a notion knocking around in my head for the past two days that Tolkien in his letters addressed this somewhere, that if Sauron had triumphed at the end of the Third Age that he would not have been left alone to rule over Middle earth forever, that either the Valar or Eru himself would have intervened to topple him in one way or another.

Correct. It's Tolkien's way of allowing for both free will and Iluvatar not allowing a permanent triumphing of evil.

But the eventual defeat of Sauron is actually presaged in Eru's chding of Melkor in the Aunulindale when Melkor tried to take over and dominate the Music of the Ainur
:

Mighty are the Ainur, and mightiest among them is Melkor; but that he may know, and all the Ainur, that I am Iluvatar, those things that ye have sung, I will show them forth, that ye may see what ye have done. And thou, Melkor, shall see that no theme may be played that hath not its uttermost source in me, nor can any alter the music to my despite. For he that attempteth this shall prove but my instrument in the devising of things more wonderful, which he himself hath not imagined.