Sauron was not able to take physical form in the LOTR. He was the "eye of sorrow". Also, I believe that his reign over middle earth would never be broken, unless Eru himself decided otherwise. Gandalf also said that if Sauron found the one ring, his victory would be swift and complete, so complete that his realm would last until the end. (thats not exact quotation, but you can look it up).
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