system
Given the following post, respond as if you're commenting on Reddit:
human
[Lord of the Rings] If Sauron getting his ring back would spell certain doom for Men, Elves and Dwarves, why was he originally defeated in the first place?
gpt
Men and elves were both stronger at the time of the Last Alliance. The elves were certainly in their autumn compared to past Ages but were more numerous and powerful than at the end of the Third Age, which is really their last hurrah. 

While Men had also declined from their height in the golden age of Numenor, they were still led by full-blooded Numenoreans, bolstered by prosperous and populous colonies that were also either Numenorean or recent descendents. While Gondor remembers those glory days, they're really just keeping the lights on and sweeping the floors.

Even with both of those, it still took a ~~year~~years-long seige to finally draw Sauron out of Barad-Dur. After his fall, his servants spent millenia weakening the Free Peoples. Moria and Erebor both fell, the elves of Mirkwood were isolated and harassed. Both Arnor and Gondor were wracked with internal and external enemies; the Northern Kingdom fell/dissolved entirely into roving bands of Rangers, while Gondor as mentioned is a shadow of itself.

On top of everything else, almost all the former alliances have broken. The Rohirrim regard the Lady of the Wood as a dangerous and vaguely hostile sorceress. Dwarves were always a bit isolationist but are now largely traveling craftsmen, never staying in any one place for long. What few elvish strongholds remain largely keep to themselves too. But even if they could present a unified front, at the end of the Third Age the Free Peoples couldn't put up a fraction of the power they did in the Last Alliance.