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human
[LotR] Would Sauron still have been able to eventually regain power if Isildur had succesfully delivered the One Ring to the Keepers of the Three rather than dying at Gladden Fields?

Normally I dislike nebulous "what-if" questions, but I intend this to be a question of the elves' capabilities more so than what would happen in the aftermath. I would think the answer would be yes, considering time was on Sauron's side given his growing forces and the continued decline of the kingdoms of Man. However, would there have been something Gandalf, Galadriel, and Elrond could have done to keep Sauron in check in the long term if they had been given the One Ring by Isildur, or would they have only.managed to further delay Sauron's rise? Alternatively, at the time of the Disaster of the Gladden Fields, could elves and man have returned to Mordor to destroy the One Ring?
gpt
>would there have been something Gandalf, Galadriel, and Elrond could have done to keep Sauron in check in the long term if they had been given the One Ring by Isildur, or would they have only.managed to further delay Sauron's rise?

Yes. Gandalf could have taken the Ring for himself and claimed it. This would have the same effect on Sauron as the Ring's destruction did. The end result wouldn't be any better though - Gandalf would have been an even worse Ring-lord than Sauron, turning good into evil. In any case, he wasn't even in Middle-Earth yet at this point. When Isildur died, the Three were held by Elrond, Galadriel, and Círdan. Gandalf was still in Valinor.

>Alternatively, at the time of the Disaster of the Gladden Fields, could elves and man have returned to Mordor to destroy the One Ring?

They could have returned, but it's unlikely they'd have been able to successfully destroy it. They'd have gotten to Mount Doom and given in to temptation, either by claiming the Ring as Frodo did or else failing in some other way.