A Face-palm for Sauron

[Farael]

I think we are approaching matters from the wrong perspective here. However, there have been so many (and so varied) ideas proposed that I'm not sure if I am merely rephrasing and repeating what others have said.

However, here are my thoughts in the matter:

-Sauron measures others perspective and thus is unable to comprehend why someone would not push their advantage to the limit. He doesn't just defeat his enemies, he CRUSHES them. See how he deals with Aragorn and the Captains of the West? He doesn't just dispatch a force strong enough to defeat them and keeps the rest in reserve for an unexpected eventuality. He lets all heck break lose upon his foes. So, since he's unable to put himself in someone else's shoes, he can't comprehend why Isildur wouldn't ruin every last vestige of him.

-Sauron could not have known for sure the effect the Ring would have had in other peoples. This wasn't a "corrupted" ring like he gave to the dwarves and men. The One Ring was more along the lines of the elven rings! Powerful on their own right and answering to no other will (except that the elven rings would've answered to the One Ring if Sauron had used it)

So, taking those two previous points it would make sense if Sauron had assumed that Isildur, being a great man among great men, had not fallen prey to the ring lure and had destroyed it, since it's what sauron would've done in his place

Now, Sauron is as sure as he can be that the Ring was destroyed. Next thing you know, he rebodies and the foundations of Barad-Dur are still there. There is no doubt in his rational mind that the ring WAS destroyed, thus evidence supports the idea that he in fact had enough innate power to survive that catastrophic event. After all, IF the ring was still around then surely someone would've claimed it and Sauron would have been able to see the clues that must've been clearly left behind.

Then he finds out the ring was NOT destroyed after all. So his certainty that he had survived the destruction of the ring disappeared into thin Mordor air. Therefore, when Frodo makes it to the crack of doom, he has every reason to worry!

After all

'And this is the dreadful chance, Frodo. He believed that the One had perished; that the Elves had destroyed it, as should have been done.

Gandalf is speaking in past tense. he BELIEVED the ring had perished. Now he knows better.