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human
Couldn't the alliance of Elves and Men have faught off Sauron even if he had the ring? They had done it in the 2nd Age and he was at full strength. Why not again?

It's mentioned that if Sauron were to get the ring from Frodo that all hope would be lost and the world would be plunged into darkness. Although the world was plunged into darkness in the 2nd age and the last alliance was able to beat him in head on combat ( at great cost of course) even with Sauron wielding the One Ring . Still though I understand Sauron would beat them separately but Elrond and Aragorn the leader of men clearly would have allied for the great battle once more. Could they not repeat their success? ( I am aware he wouldn't be beaten for good without destroying the ring but they could at least beat Sauron and his forces head on)
gpt
The strength of Elves and Men was not what it had been. The Elves never replaced their losses in the War of the Last Alliance, which had been severe. In addition, over the 3,000 years since, many of the Elves had left Middle-Earth for Valinor. This was especially true of the Noldor, the High Elves, who were the greatest and mightiest of them. Very few remained in Middle-Earth during the time of the War of the Ring.

And Men have waned, as well. The Men of the Last Alliance were still the Men of Numenor undiminished, with all their knowledge, pride, and strength. 3,000 years of mingling their blood with lesser Men meant that strength was all but gone. And at the same time, forces loyal to Sauron had been waging a constant war to weaken and reduce the remnants of vanished Numenor. Arnor had been destroyed completely by the Witch-King; all that was left of that great realm were the scattered Rangers of the North. They were valiant, but they were very, very few. Gondor endured, but her population was greatly reduced by invasion and plague.