Eru tripped Gollum, according to the LOTR wiki, thus destroying the One Ring. If he wanted it destroyed, why didn't he do something earlier, when Isildur took the Ring for his own when he was at the Cracks of Doom (if you take the movie for canon)? He could have tripped Isildur, or if that isn't honour-worthy enough for the son of Elendil, made the Ring slip from his fingers or something similar.
Why did he have to wait for an age to make it fall?
Well, this has been sticking in my craw for a while, so I better get it out.
This question is based in the premise that Ilúvatar "tripped" Gollum at the Cracks of Doom, making it "inconsistent" not to trip Isildur the last time the Ring was there. But
Ilúvatar didn't trip Gollum.
My heart tells me that he has some part to play yet, for good or ill, before the end; and when that comes, the pity of Bilbo may rule the fate of many -- yours not least.
-Gandalf to Frodo, Book I, chapter 2, "The Shadow of the Past".
"Making Gollum trip" is a common, but incorrect, over-interpretation of this passage in Tolkien's letter (#192) to Amy Ronald on 27 July 1956:
Frodo deserved honour because he spent every drop of his power. The Other Power then took over: The Writer of the Story (by which I do not mean myself), the one Person who is never absent and never named.
But this is what Tolkien actually wrote in the book:
And with that, even as his eyes were lifted up to gloat on his prize, he stepped too far, toppled, wavered for a moment on the brink, and then with a shriek he fell.
-Book VI, chapter 3, "Mount Doom"
Gollum, in his elation over having finally gotten the Ring back, wasn't paying attention to his surroundings and stepped over the edge.
We do have these interesting interactions with Gollum in Book IV:
"Sméagol will swear on the Precious".
Frodo drew himself up, and again Sam was startled by his words and his stern voice. "On the Precious? How dare you?" he said. "Think"!
One Ring to rule them all and in the darkness bind them.
Will you commit your promise to that, Sméagol? It will hold you. But it is more treacherous than you are. It may twist your words. Beware!"
-Frodo and Gollum, Bool IV, Chapter 1, "The Taming of Sméagol"
Later:
If I, wearing it, were to command you, you would obey, even to leap from a precipice or cast yourself into the fire. And such would be my command.
-Frodo to Gollum, Book 4, Chapter 3, "The Black Gate is Closed"
And finally, after Gollum's attempt to get the Ring on the slopes of Mount Doom:
Then suddenly, as before under the eaves of the Emyn Muil, Sam saw these two rivals with other vision. A crouching shape, scarcely more than the shadow of a living thing, a creature now wholly ruined and defeated, yet filled with a hideous lust and rage, and before it stood stern, untouchable now by pity, a figure robed in white: but at its breast it held a wheel of fire. Out of the fire there spoke a commanding voice:
"Begone and trouble me no more! If you touch me ever again, you shall be cast yourself into the Fire of Doom."
-Book VI, Chapter 3, "Mount Doom"
The first two are possibly mere foreshadowing. The third is a little more important. First, this is apparently the point where Frodo was pushed over the edge into wielding the Ring. Second, it appears the Ring itself, not Frodo, is warning Gollum what would happen if he touched it again. And this is exactly what happened.
Letter 192 was written in response to a comment that Frodo was somehow "bad" for finally giving in and claiming the Ring for himself. But Frodo had spent all of his strength just getting to the Cracks of Doom. The only way the ring was going to go into the Fire was from someone who craved it just as much as Frodo did, and was heedless of the consequences.
So, how and where did the Maker of the Story intervene? It was all the way back in the Hobbit, when pity stayed Bilbo's hand from slaying Gollum.
This excellent Youtube video by In Deep Geek helped focus my thoughts.
Free will. Eru never compels creatures -- be they Valar or Hobbits -- to act other than according to their own choices. Even the Valar freely choose to submit to Eru's will -- Melkor fell precisely because he did not so choose.
Gandalf chose to be embodied and go to Middle-Earth and try to rouse (but not force) Men and Elves to resist Sauron. Aragorn chose a life of hardship with little hope to resist Sauron. Faramir and Galadriel chose to renounce the Ring when they had it in their grasp. The Hobbits chose to go off on a dangerous journey with little chance of success.
Likewise, Boromir and Denethor each chose to try to seize the Ring -- having been warned that that would be terribly dangerous and ultimately worse than futile -- to save Gondor. Saruman chose to turn away from his task to pursue power. Sauron and Melkor chose to selfishly pursue power over others.
Each of them had the free will to choose good or evil based on their own values, personalities, wisdom (or lack of wisdom), knowledge, motives -- in short, based on themselves.
Isildur took the same path as Boromir and Denethor and in spite of knowing the danger of the Ring, chose to keep it. For Eru to have tripped Isildur would have been to make a mockery of his free will.
Finally, in the Silmarillion, Eru says of Melkor's evil (but free) choices:
And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.'
In the end, even creatures' evil choices will result in a better creation.
Finally, did Eru trip Gollum? He certainly didn't do so 'on screen'. The story doesn't say and no one in the story knows. Gandalf certainly doesn't. OTOH, Gandalf certainly believes in what we call "providence" but he believes in it and doesn't know it for a fact. On another matters, he says to Frodo:
"...It was the strangest event in the whole history of the Ring so far: Bilbo's arrival just at that time, and putting his hand on it, blindly, in the dark.
'There was more than one power at work, Frodo. The Ring was trying to get back to its master. It had slipped from Isildur's hand and betrayed him; then when a chance came it caught poor Déagol, and he was murdered; and after that Gollum, and it had devoured him. It could make no further use of him: he was too small and mean; and as long as it stayed with him he would never leave his deep pool again. So now, when its master was awake once more and sending out his dark thought from Mirkwood, it abandoned Gollum. Only to be picked up by the most unlikely person imaginable: Bilbo from the Shire!
'Behind that there was something else at work, beyond any design of the Ring-maker. I can put it no plainer than by saying that Bilbo was meant to find the Ring, and not by its maker. In which case you also were meant to have it. And that maybe an encouraging thought.'
Gandalf is surmising -- he "can put it no plainer" because he doesn't know. He believes and he hopes, but he doesn't know.
Later addition: The whole matter of free will and providence is very subtle, and I think Tolkien understood that subtlety. Eru knew that Gollum would carry the Ring into the fire, but He didn't push Gollum: He didn't have to.
Gollum became Gollum as the consequence of a lifetime of bad choices deliberately made by Gollum (and by Smeagol before him.) Smeagol was captured almost instantaneously by the Ring and killed his friend Deagol to possess it. Had Smeagol been a different person, he might have resisted -- both Bilbo and Frodo did, after all. So did dozens of other people (Men Elves, Dwarves, and Hobbits) who knew of it and had an opportunity to seize it.
Gollum had turned himself into just the sort of person who would fight Frodo at the edge of the chasm at Mt Doom and who would be so enthralled by the Ring that he would disregard everything around him (including a hole filled with molten lava...) once he had it. Dancing in glee, oblivious, he fell.
Eru knew that would happen, but it still happened entirely as a consequence of choices made by Gollum based on his own personality, desires and experience. Gollum was no one's puppet. (If Eru did anything, he -- almost miraculously -- preserved Gollum until he could finally, by overreaching in a way completely in character, destroy the Ring.)
