Qadgop, I’m not disagreeing with you, because I know you’re a mighty Tolkien scholar, and I’m but a fool of a billybumbler. But what is the source for this, and does it make sense? How could Sauron be defeated while wielding the Ring by Children of Iluvater at the end of the Second Age when the Ring was so powerful by the end of the third age that the Children of Iluvater plus a Maia couldn’t so much as touch Sauron while he sort-of drew on its power from afar? I don’t see how Sauron could be defeated on the Fields of Gladden except by Isildur’s action. Again, I’m not saying you’re wrong; I just want to understand where you’re getting this from and how it can be.
From the appendices in ROTK:
And from Sil:
And from HOME VI:
HOME VIII says:
So we see that JRRT definitely stated in many different places that Sauron was indeed overthrown by the two kings, not by Isildur. I tend to believe Sauron wasn’t slain, but was cast down by this action, and that it was the removal of the ring by Isildur that caused him to surrender visible shape.
As for why the Children of Iluvatar alone couldn’t throw down Sauron by force of might again at the end of the 3rd age, from Sil:
Remember, The Last Alliance contained many High Elves, who were mighty on the earth, mightier than their Sindarin brethren like Legolas. And it also contained a lot of Sindar, who were in turn far mightier than their sylvan kin like Haldir.
It also contained many men of Numenor descended from Elros, the brother of Elrond, with blood unmingled with the ‘lesser’ folk of middle-earth. The nation of Arnor was large and strong, as was the nation of Gondor. The dwarves of Durin’s line also fought alongside elves and men against Sauron, as they’d done against Morgoth himself in Dagor Bragollach and Nirnaeth Arnoidiad.
Sauron’s orc armies were also lesser in those days, without the Uruk-hai, those uber-orcs who appeared only midway through the 3rd age. And Sauron had fewer human allies, as the peoples of Umbar and Harad and Rhun had long been intimidated by Numenorean (and later Gondorian) might.
So, to summarize:
The Last Alliance had lots of Noldorin and Sindarin (and mixed) elves, long steeped in both wisdom and battle, and unfading. Lots of Dunedain, especially pissed at Sauron for what had happened to Numenor. Lots of Dwarves, whom you do not want to piss off. All opposing Sauron, who’d only recently returned to Middle-Earth after the wreck of Numenor and the loss of his body, both of which were unanticipated setbacks for him. He also had relatively few slaves/supporters, all of which he spent lavishly in the battles before coming to to do battle in person.
The War of the Ring had the remnants of Gondor, long winnowed by battles with enemies from Umbar, Harad, and elsewhere, slowly withdrawing inward. A handful of Rangers from what was once Arnor. And the Rohirrim, recently decimated by the orcs of Saruman and the depradations of the Dunlendings. This was arrayed against the forces Sauron had been accumulating slowly for centuries. Heck, Sauron didn’t need the ring on his hand to wipe out this tiny bit of opposition.
Oh, and remember that the Maia Gandalf wasn’t allowed to be Sauron’s primary foe. His duty was to kindle the hearts of men to resist Sauron.
Thanks, Qadgop! That was exactly what I was looking for.
As long as I’ve got you, could you tell me why Galadriel went and Celeborn stayed? I mean, Celeborn seems to have then gone a few decades later, and doesn’t seem to me to have done anything all that important while he remained, so why didn’t he go with her in the first place? He seems a bit bitter about losing her.
Remember, elves are different. Reliving a memory is far closer to reality for them than it is for mortals.
Also, Celeborn knew their parting would not endure beyond the end of the world. I suppose he fully expected to join her in the Undying Lands within a few decades or centuries, or whatever. They’d had a few millenia together already, what’s a little time apart? Lots of couples take separate vacations from time to time.
That was my take on it, anyway. Their relationship has always been somewhat ambiguous and frequently re-written by JRRT anyway. Was Celeborn a Teleri from Alqualondë or a Doriathrin from Beleriand? His origins kept changing.
Besides, for a guy whose original name was Teleporno, a few quirks are to be expected. 
Yes, but doesn’t he say to Aragorn something along the lines of “I hope you keep my treasure, unlike me, losing mine?” That sounds involuntary and a tad bitter to me.
I think he was speaking of his granddaugher, Arwen. Or perhaps his Daughter, Celebrien.
But maybe not.
Oh. I never thought of that. You may be right.