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human
[LotR] As a Maia, how was Sauron finally and truly defeated? Couldn't he just bide his time and slowly recoup his power as he had done several times before?

As I understand it, a Maiar's physical form is like clothing they put only a small amount of their power into and it can be shed at will or if circumstances demand. We see this with Saruman in The Scouring of the Shire when his spirit separates from his body. Did Sauron simply put all of his power into the Ring, making its destruction his total destruction as well? Surely he wouldn't have put all of his power into one physical object so easily separated from him. It's been awhile since I've gone over the stories, so forgive me if I'm misremembering anything.
gpt
> Did Sauron simply put all of his power into the Ring, making its destruction his total destruction as well? 

Pretty much. Sauron was a being consumed with the concepts of domination and control. The ring was his ultimate attempt at dominating the minds of some of Middle Earth's most powerful individuals, thereby making him the de-facto ruler of the world even if no one realized it. But in order to make something that powerful, he had to pour a great deal of his "essence" into it. It was as much a part of his being as his head or his spine, and he was only whole while it was with him. 

With the ring destroyed, a piece of Sauron had been permanently severed from him. His spirit cannot die, and it continues to exist to this day as a lingering fog of malice and hate, but he no longer has the power to reform a new body on his own. 

> Surely he wouldn't have put all of his power into one physical object so easily separated from him.

Sauron never thought that it would be separated from him. Sauron has never been a front-line leader, wading into the middle of a battle where he might be injured. His presence during the battle where he was last defeated was at the very end of a seven-year-long siege, and he had completely run out of options. If things went according to plan, Sauron would never leave the safety and comfort of his fortress, and the ring would stay safely on his finger. 

Destroying the ring was even more unthinkable. Even in the event of it being separated, Sauron knew that the ring would seek to return to him. After all, it is a small piece of his very existence, and it would want to rejoin the bigger fragment. Sauron and the majority of people in Middle Earth who knew the ring's truth assumed that no one would EVER think to unmake it. Its pull toward power and control is too strong. 

That's why the plan to sneak into Mordor and drop the ring into the Crack of Doom was so extreme. It would be like trusting the defeat on your greatest enemy by sending a gnat into their biggest military base while carrying an armed nuclear warhead. It just wouldn't logically work according to all the known rules.