sauron winning?

[Pitchwife]

Originally Posted by Mnemosyne

For me the issue would not so much be the Valar's inability to help--but the fact that, if they did help, the Third Age would end just the same as the First and Second--with a cataclysm of epic proportions that would hurt the very people it was trying to help.

My first thought was something quite similar - as Sauron's dominion in the Third Age extended over a much larger part of Middle-earth than Morgoth's in the First, any intervention by the Valar along the lines of War of Wrath Reloaded might have escalated to a continent-wide war which would lay most of the habitable world in ruins. On second thought I realized that they wouldn't actually have had to militarily scour all Rhûn and Near & Far Harad - all they had to do was to get through to Barad-dûr and deal with Sauron himself, which should have been feasible, especially if they acted promptly, while there still was some resistance to him in the West and they wouldn't have had to fight their way all the leagues from the Havens. Once Sauron was overcome, his misled human followers could have been reeducated and (to use a 20th century metaphor) 'denazified', like the Dunlendings after Helm's Deep and the fall of Orthanc.
The bigger problem is that, while Morgoth's forces in the First Age consisted chiefly of Orcs, Trolls, dragons and fallen Maiar, Sauron's in the Third comprised whole nations of Men, and I think we can take it for granted that he would have thrown every available bit of cannon-fodder into their way, so the Valinorean Cavalry could (and probably would) have been facing the necessity to slaughter Ilúvatar's Younger Children by hundred thousands - hurting the very people they were trying to help, as Mnemo put it so well. And as we learn from the Fall of Númenor, the Valar felt the Second-Born were under Eru's personal jurisdiction, so His permission would certainly have been needed.
But even if it was given, this would have been an extremely desperate measure. As Legate and Mnemo said, Mortal Men (including Hobbits) needed to grow up and learn to deal with evil in whatever guise themselves, without overt help from Higher Powers.