![[info]](https://www.insanejournal.com/img/userinfo.gif){kind=small}
It's the combination of realizing the Harrycrux and the second half of the prophecy, the one Severus didn't hear.
Because the prophecy speaks of (at least) 2 encounters between Tom and Harry - in the first Tom will mark Harry as his equal and in the second 'either must die at the hand of the other'.
It is the prophecy that made Albus realize the meaning of Harry's scar - why does the scar make them equal? Because they both contain Tom's soul. And that is also why Harry was the one with the power to vanquish Tom - not because of any talent of his (he didn't have any that was relevant) but because his death could destroy Tom (well, once the other Horcruxes were gone, but Albus only learned of them later).
If Dumbledore knew about Horcruxes this early, he'd've gotten to 'em before Harry even got to Hogwarts, way I figure it.
Albus knew what Horcruxes were by the 1940s. Even before he became headmaster. He banned the topic and confiscated the books (Horace tells young Tom that it was specifically Albus who banned the teaching of the topic).
He knew about Tom's collection of trinkets whenever it was that he spoke to Morfin and Hokey, probably in the 1950s (because I can't see them surviving for many decades after their respective framings, and there were plenty of leads Albus could have followed). He did not know to what purpose Tom collected the objects until Harry returned from the graveyard and said that Tom spoke of multiple experiments in immortality, of going beyond anyone else before him down that path.
But he knew Harry was a Horcrux from day one. Because he saw the scar, and he figured out that Harry was saved because of Lily's sacrifice. So he knew Tom's AK couldn't have possibly hit Harry. Yet something did - hence the scar. He realized that what hit Harry was Tom's soul.
Because the prophecy speaks of (at least) 2 encounters between Tom and Harry - in the first Tom will mark Harry as his equal and in the second 'either must die at the hand of the other'.
It is the prophecy that made Albus realize the meaning of Harry's scar - why does the scar make them equal? Because they both contain Tom's soul. And that is also why Harry was the one with the power to vanquish Tom - not because of any talent of his (he didn't have any that was relevant) but because his death could destroy Tom (well, once the other Horcruxes were gone, but Albus only learned of them later).
If Dumbledore knew about Horcruxes this early, he'd've gotten to 'em before Harry even got to Hogwarts, way I figure it.
Albus knew what Horcruxes were by the 1940s. Even before he became headmaster. He banned the topic and confiscated the books (Horace tells young Tom that it was specifically Albus who banned the teaching of the topic).
He knew about Tom's collection of trinkets whenever it was that he spoke to Morfin and Hokey, probably in the 1950s (because I can't see them surviving for many decades after their respective framings, and there were plenty of leads Albus could have followed). He did not know to what purpose Tom collected the objects until Harry returned from the graveyard and said that Tom spoke of multiple experiments in immortality, of going beyond anyone else before him down that path.
But he knew Harry was a Horcrux from day one. Because he saw the scar, and he figured out that Harry was saved because of Lily's sacrifice. So he knew Tom's AK couldn't have possibly hit Harry. Yet something did - hence the scar. He realized that what hit Harry was Tom's soul.