From the Letter of JRR Tolkien, pages 332-3 (draft letter to Mrs Eileen Elgar, September 1963):
"...only Gandalf might be expected to master [Sauron] - being an emissary of the Powers and a creature of the same order, an immortal spirit taking a physical form. (...)
"If Gandalf proved the victor, the result would have been for Sauron the same as the destruction of the Ring; for him it would have been destroyed, taken from him for ever.
"But the Ring and all its works would have endured. It would have been the master in the end.
"Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained 'righteous', but self-righteous. He would have continued to rule and order things for 'good', and the benefit of his subjects according to his wisdom (which was and would have remained great)."
[The draft ends here. In the margin Tolkien wrote: "Thus while Sauron multipied [illegible word] evil, he left 'good' clearly distinguishable from it. Gandalf would have made good detestable and seem evil."]
"Gandalf would have made good detestable and seem evil." This state of affairs has actually happened in this world, recently and still now, under totalitarian Communism. And we are seeing it becoming ever more the case, incrementally and quickly, throughout the West.