Canon question (DH)...
When Voldy had just killed Harry's parents, leaving Harry orphaned, and Dumbledore is speaking with Snape about needing to protect Harry...
Snape is refusing, telling Dumbledore that the Dark Lord is gone for good and that Harry does not need protection.
Dumbledore tells Snape, instead, that Voldy will be back.
Question:How does Dumbledore know that Voldy is not defeated for good?
It can't be the prophecy that Dumbledore heard, b/c Snape eaves-dropped on it and he is not convinced that Voldy is coming back.
Snape is refusing, telling Dumbledore that the Dark Lord is gone for good and that Harry does not need protection.
Dumbledore tells Snape, instead, that Voldy will be back.
Question:How does Dumbledore know that Voldy is not defeated for good?
It can't be the prophecy that Dumbledore heard, b/c Snape eaves-dropped on it and he is not convinced that Voldy is coming back.
It's probably this. Snape only heard a part of the prophecy, you're so right!!!!!! Forgot about that. But Voldy *did* spend a nice amount of time in Harry's 5th year trying to figure out what it was Dumbledore knew that he didn't. (LoL - what Dumbledore knows that Voldy doesn't can fill a book!^^).
As for knowing about Horcruxes... it seems that Dumbledore had figured the Horcruxes thing out in Harry's 2nd year, when Harry gives him a defeated Horcrux. I think if Dumbledore knew about Horcruxes earlier, he'd've started a bit earlier to find and destroy them, as in, before Harry is even in Hogwarts.
I like the 'full prophecy' explanation -- that'd probably do it. And it explains what it is Dumbledore knows that Snape doesn't. ;)
And Albus' entire treatment of Harry - from leaving him on the front steps on a November night, through leaving him with the Dursleys despite Arabella's reports, through choosing Hagrid to be the one to deliver the letter to Harry and everything else was done because Albus was planning for Harry to die and he needed to brainwash him into unquestioning obedience so that he would do so.