Hee.
Well, so there wasn’t anyone in Middle Earth with the power of Sauron, was there? How did this even happen? How come there wasn’t an opposite, so to speak? What were the Gods of ME doing, or do they not have that kind of power?
I think that the gods had some sort of non-interference policy or something. Their only intervention was to send the wizards.
He’s a powerful Maia, one of the spirits dating from the creation of the world; think “fallen angel”. Gandalf is a similar but less powerful Maia; so was the Balrog. The Valar are much more powerful; think “archangel”, but they tend to break continents when they get involved. They destroyed Numenor and Sauron’s old body, along with his ability to take on a pleasing aspect; so yes, they are stronger than him. But they’ve decided not to intervene directly, which probably for the best considering their track record of collateral damage. As for God aka Eru Illuvatar, everything is going according to his plan; it’s that kind of theology.
Sauron is actually the leftover lieutenant of the original Big Bad Guy, Morgoth aka Melkor, who is a Valar not a Maia and basically the equivalent of Satan. He greatly weakened himself by pouring his essence into the world to corrupt and control it, and was eventually thrown out of the world into the Void until the end of the world. Note that he couldn’t be destroyed because he had done with the world much what Sauron did with the Ring, infused most of his essence into it; the world of Arda is therefore sometimes referred to as the Ring of Morgoth because Morgoth could only truly be destroyed by destroying the world.
And as said, drawing upon this “Morgoth element” is part of what makes the Ring powerful.
So…they wouldn’t interfere with Sauron messing up the world because they figured they would mess up the world? How did Sauron reign in his destructive tendencies, then? Sure, Mordor was a sucky place, but it still existed and millions of people (orcs) lived there. It wasn’t destroyed.
Or at least why didn’t they send another equally powerful Maia to combat Sauron? If the answer is “they just didn’t for the sake of the story” that certainly is an acceptable answer! But if there is one in the story I’d like to know it.
I’m just guessing at this point, but I believe that there was no Maia as powerful.
OK, maybe Tom Bombadil, but he couldn’t be arsed.
Sauron was all about Imposing Order. He didn’t want to destroy the world, just control and order it.
As I understand it, they decided that Middle Earth should solve the problems of Middle Earth, so they sent the Wizards - Maia with restrictions on their powers and actions - to help and advise, rather than to fight Sauron directly. Which is why Gandalf the White is stronger than Gandalf the Grey; he was sent back with looser limits.
Just so. Plus, anyone equally powerful in direct conflict would tend to… damage… things. A lot of things. And there may not have been anyone that badass since Ar-Pharazon the Golden. Sauron took one look at him and said, “Mai gib up!”
Seriously, though, They didn’t really have anyone to send, and interfering directly does not have a good history for the Valar. Most contact from lands to Middle-Earth did not go well in the long run, and they know it’s liable to hurt the very mortals they wanted to help. Thus, a minimalist approach: send a Maiar. Don at the bottom of it, no matter what Sauron had given to him from Melkor, he was still a Maiar.
Saruman may have actually discovered a pure way of Ringmaking. He definitely had his own, and was apparently not controlled by Sauron. In fact, he was planning his own tricks the whole time, trying to get the Ring for himself. Like Galadriel, he had maybe a 50/50 shot at crushing Sauron. Then he’d take Sauron’s power and place as Dark Lord.
If I recall one of Morgoth’s specialties was crafting. Metal crafting. He taught some of what he knew to the elves. (He coveted the Elves finest work of craftsmanship, the Simarils. He stole them for a while, starting a war that ended with the sundering of the world at the end of the First Age.)
Sauron was also a master craftsman, and passed the art of ring making on to the elves, dwarves, and men. However, I think he might have passed on deliberately bugged/tainted knowledge. When the children races crafted their rings, Sauron had a “back door” way into the minds of the wearers.
It’s pretty clear that he did make a Ring (how powerful it was is anyone’s guess), but I think it’s a stretch to say that it was pure. Remember, at no point in the existence of Saruman’s ring did Sauron have the One, so direct control was never tested, and certainly Saruman ended up corrupted, which may have been in part due to the Ring he made.
-Book III, Chapter 8. He only thought he was independent.
Tom Bombadil was not a Maia. Also, Gandalf, who while not infallible is as close to the Voice of God as we’re going to get within LOTR, says that even Bombadil’s realm would eventual fall if Sauron proceeded unchecked.
He would be “last as he was first” was the line, as I recall.
Note also that there’s not just one dimension of “greatness”, here. While Sauron was certainly more powerful than Gandalf, and probably the most powerful of the Maiar, it’s also explicitly stated that Gandalf was the wisest of the Maiar.
From a rec.arts.books.tolkien posting dated 19 July 1995:
What IS it with Aule’s maiar, anyway? Saruman AND Sauron were servants of Aule before they went bad. I wonder if it has something to do with Tolkien’s demonstrated hostility to industry? Industry is destroying the natural world, Aule is the Vala of industry, therefore his maiar are more prone to corruption?
ETA: For that matter, Feanor (and the Noldor in general) was a favorite and protege of Aule as well. Hmmmm…
That’s definitely an influence, but also from the other side, that the corrupt are the kind of poeple who will be attracted to fabricating dead things into tools and weapons.
People with a mind for making things (sub-creations) tend to forget Creation in favor of their sub-creations, and thus fall. Same with Melkor.
A lot of what people have said in this thread strikes me as correct: that Melkor during the big song introduced evil and implanted part of himself and that evil into the Creation (as Illuvatar intended) and there was inherent power of Melkor in Arda. The ring is able to tap into this power.
What I haven’t seen in this thread is quotes from the chapter The Council of Elrond: “the treasure of the Enemy, fraught with all his malice, and in it lies a great part of his strength of old.” Fraught in this sense is 3fraught 'frȯt\ adj [ME, fr. pp. of fraughten] (14c)
1 archaica : laden b : well supplied or provided
2 : full of or accompanied by something specified — used with with ‹a situation ~ with danger›
3 : causing or characterized by emotional distress or tension : uneasy ‹a ~ relationship›
Here is a thoughtful web page on the subject 4. The Meaning of the Ring
I also seem to recall language that “he poured all his malice and will to dominate into the ring”, but that may be just from the movies.
I’d say that the inclination to shape and order things for material/craftsman oriented Ainu places them in a position that, when combined with pride, can lead to … poor choices. 
Actually, we’re not sure exactly who/what Bombadil was.
Since he was the creation of J.R.R. Tolkien, and Tolkien says he was an enigma even to the author, any assertion that Tom Bombadil was definitively an X is untrue. Besides, it is written that “Tom remembers the first raindrop and the first acorn…he knew the dark under the stars when it was fearless — before the Dark Lord came from Outside.” He was in the world before the Maiar.