3

When Voldemort planned to kill baby Harry, Snape leaked this to Dumbledore, who in turn alerted the Potters, which finally cost Voldemort his body. If the Potters didn't know beforehand, he would have had an element of surprise, or he could have sneakily killed Harry.

Voldemort definitely knew that Snape leaked the information because Pettigrew was his inside man.

Why did he then trust a traitor after his return?

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    Uhhh.... the Potters knew beforehand? And they cunningly waited for him and died? I don't think so. – Radhil Jul 21 '18 at 13:57
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    @Radhil Of course they knew he wanted them dead! It's the whole reason they started using the fidelius charm in the first place. – ZenLogic Jul 21 '18 at 14:36
  • The Potters were hidden by the Fidelius charm with Pettigrew as secret keeper. The prophecy referred to one boy born at the close of July (or summer?) which could have been Harry or Neville. I thought Snape found the aftermath...? – Pam Jul 21 '18 at 14:38
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    @Pam Snape asked Dumbledore to hide them with the Fidelius charm, when he found out the Dark Lord interpreted the prophecy as referring to Harry. – ZenLogic Jul 21 '18 at 14:40
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He didn’t know

There is no evidence to show that Voldemort ever knew Snape had betrayed him by revealing his plans to Dumbledore; on the contrary, there is a fair amount of circumstantial evidence to indicate that he didn’t. For one thing, if he had known, Snape would almost certainly have died before Philosopher’s Stone even begins, and all seven books would have been very different.

Voldemort definitely knew that Snape leaked the information because Pettigrew was his insider man.

Your logic does not really hold water here.

As QuestionAuthority’s answer says, Voldemort knew that Dumbledore had heard the entire prophecy—and thus knew more than Snape was able to tell Voldemort. He also knew, through Pettigrew, that the Potters were hidden under a Fidelius Charm. But that was all he knew.

It so happens that the Fidelius Charm probably came about mainly because Snape told Dumbledore that Voldemort had interpreted the prophecy to refer to the Potters, not the Longbottoms—but how would Voldemort have known about this? There are only really two ways Voldemort could have found out: from his spy Pettigrew, or from Snape himself.

That Snape didn’t volunteer the information to Voldemort himself does not need much by way of convincing argumentation. He would know that doing so would be signing his own death warrant. No need to even entertain the notion.

And Pettigrew almost certainly couldn’t have told Voldemort, because there’s no reason to believe he ever knew. As soon as Dumbledore found out about the Potters being targeted, he used Snape’s feelings for Lily to make him turn spy against Voldemort, which was a very valuable asset to him. He would obviously not do anything to jeopardise it if he could help it, and telling anyone—even those closest to him—would be doing just that. So he almost certainly never told anyone else that Snape was now a spy for him, including Pettigrew, and Pettigrew never knew.

 

What Pettigrew knew

Pettigrew knew that Dumbledore wanted the Potters protected, and he might even have suspected that this was based on spy intel that Dumbledore had received about Voldemort’s plans. But the only way he could have known that the intel came from Snape would be if Dumbledore had told him (or if Snape had, but the point two paragraphs up holds true here as well), which would have been a very unwise and un-Dumbledorish thing to do.

 

What Voldemort knew

Voldemort in turn knew the first half of the prophecy from Snape. He knew from his own knowledge of the wizarding world and his spies (presumably) that the prophecy could refer to either the Potters or the Longbottoms, and he knew from Snape that Dumbledore knew this too (if not more). He also knew from Pettigrew that the Potters had been Fideliussed away on Dumbledore’s orders.

He could not know, however, why Dumbledore had chosen to protect the Potters, nor whether he had taken (or was taking) steps towards protecting the Longbottoms in a similar manner. He also did not know whether the final part of the prophecy had made it clear that it did indeed refer to the Potters and not the Longbottoms, so that only the Potters needed protection. All he knew was that he selected the most likely family, and this family was protected by Dumbledore.

He was clever enough to know that Dumbledore had spies within Death Eater ranks just as Voldemort himself had spies within Phoenix ranks. Even if he did ever suspect that Dumbledore’s decision to protect the Potters was borne out of spy intel, rather than simply based on the information he himself already had available to him, there is nothing that would indicate that Snape in particular was the source of that information.

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He probably didn’t know Snape told Dumbledore of the prophecy.

Pettigrew was the Dark Lord’s spy in the Order of the Phoenix, but Dumbledore didn’t tell them the whole truth when it was better that things be kept as quiet as possible. We know that Dumbledore avoided telling Harry that Snape was the one who’d heard the prophecy - Harry only found out because he heard about it from Trelawney, who didn’t even know she made a prophecy. The Order didn’t know that it was Snape either - they only found out when Harry told them.

“I’d love to know what Snape told him to convince him,’ said Tonks.

‘I know,’ said Harry, and they all turned to stare at him. ‘Snape passed Voldemort the information that made Voldemort hunt down my mum and dad. Then Snape told Dumbledore he hadn’t realised what he was doing, he was really sorry he’d done it, sorry that they were dead.’

‘And Dumbledore believed that?’ said Lupin incredulously. ‘Dumbledore believed Snape was sorry James was dead? Snape hated James …”
- Harry Potter and the Half-Blood Prince, Chapter 29 (The Phoenix Lament)

Dumbledore didn’t tell the Order, so Pettigrew wouldn’t have found out as a spy that Snape was the one who told Dumbledore of the prophecy. Therefore, Pettigrew couldn’t tell the Dark Lord, and it’s unlikely that the Dark Lord would have found out Snape was responsible for it in any other way.

The Potters being protected wouldn’t be particularly unexpected.

We know that Snape told the Dark Lord everything he heard of the prophecy. Presumably, that would include that it was made in front of Dumbledore, meaning the Dark Lord would know that Dumbledore would have heard the prophecy himself.

“The – the prophecy … the prediction … Trelawney …’

‘Ah, yes,’ said Dumbledore. ‘How much did you relay to Lord Voldemort?’

‘Everything – everything I heard!’ said Snape. ‘That is why – it is for that reason – he thinks it means Lily Evans!”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)

The part the Dark Lord heard, and would have known Dumbledore heard, said that the boy who could defeat him would be born in July to parents who defied him three times. From that, he was easily able to figure out the two boys who fit the prophecy, and likely wouldn’t be surprised that Dumbledore could have reasoned out who they were and taken measures to protect them.

“My – our – one stroke of good fortune was that the eavesdropper was detected only a short way into the prophecy and thrown from the building.’

‘So he only heard –?’

‘He heard only the beginning, the part foretelling the birth of a boy in July to parents who had thrice defied Voldemort.”
- Harry Potter and the Order of the Phoenix, Chapter 37 (The Lost Prophecy)

That the Potters were protected wouldn’t be particularly surprising or unexpected to the Dark Lord, knowing Dumbledore had heard the prophecy, so he’d have no reason to believe that their protection was the result of Snape telling Dumbledore rather than Dumbledore’s own reasoning.

3

Dumbledore heard the full prophecy from Trelawney. He also knew that Snape heard a part of the prophecy and was caught by his brother Aberforth. This part was

The one with the power to vanquish the Dark Lord approaches.
Born to those who have thrice defied him, born as the seventh month dies.

It is no stretch of the imagination that Dumbledore immediately realized that Voldemort would try to eliminate this perceived danger. As the prophecy was told before Harry and Neville were born, Dumbledore didn't immediately know who this vanquisher was going to be, but after they were born it became clear. So protecting the Potters has nothing to do with Snape leaking information.

Voldemort in turn knew that Dumbledore heard the prophecy in person, because Snape told him. So there is no reason to suspect Snape because Dumbledore wanted the Potters (and Longbottoms) protected.

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    But protecting the Potters did have something to do with Snape leaking information: he was the one who told Dumbledore that Voldemort was going after the Potters and not the Longbottoms. Without that, Dumbledore would have had to try to protect two families, which would have been much more difficult to pull off. – Janus Bahs Jacquet Jul 21 '18 at 15:57
  • We don't know that Dumbledore didn't want the Longbottoms protected. We also don't know that Voldemort didn't target the Longbottoms. We know/assume that Snape didn't care about the Longbottoms, so he didn't ask Voldemort to spare them or Dumbledore to protect them. I know the quote where Snape says "Hide them". But do you suggest Dumbledore wouldn't protect members of his order unless Snape told him to do it? – QuestionAuthority Jul 21 '18 at 16:52
  • True, we don’t know that Voldemort would not also have targeted the Longbottoms; we only have Snape’s word for that (“He thinks it means Lily Evans”). There’s nothing to suggest that he did/would, though. We only know that Voldemort decided to target (at least) the Potters, that he himself believed that Harry was the vanquisher when he went to their house, that Snape told Dumbledore about Voldemort’s choice, and that Dumbledore chose to protect the Potters. It is at least strongly hinted at, though, that it was Snape’s information that led to Dumbledore hiding the Potters. – Janus Bahs Jacquet Jul 21 '18 at 16:59
  • @JanusBahsJacquet This just shows Snape's bias. Voldemort didn't think it means Lily Evans. He didn't even think it means Lily Potter, or James Potter. He thought it might be Harry Potter or Neville. But Snape doesn't care about any of them, except Lily Evans (not Potter). – QuestionAuthority Jul 21 '18 at 17:12
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    We will never know for sure, but again: there is nothing at all in the books that indicates that Voldemort had any plans involving Neville at all. If he planned on killing Harry first and then going after Neville afterwards, why think of Harry as “this one, inexplicable danger” and spare no thought at all for Neville? Everything in the books points to Voldemort having decided—for whatever reason—that the prophecy must refer to Harry, not Neville. – Janus Bahs Jacquet Jul 21 '18 at 18:04
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Voldemort didn't know Snape was the trator

No one really knew Snape had leaked the secrets to Dumbledore. Here, Bellatrix lists why she does not trust Snape:

"Narcissa, I think we ought to hear what Bellatrix is bursting to say; it will save tedious interruptions. Well, continue, Bellatrix," said Snape. "Why is it that you do not trust me?"

"A hundred reasons!" she said loudly, striding out from behind the sofa to slam her glass upon the table. "Where to start! Where were you when the Dark Lord fell? Why did you never make any attempt to find him when he vanished? What have you been doing all these years that you've lived in Dumbledore's pocket? Why did you stop the Dark Lord procuring the Sorcerer's Stone? Why did you not return at once when the Dark Lord was reborn? Where were you a few weeks ago when we battled to retrieve the prophecy for the Dark Lord? And why, Snape, is Harry Potter still alive, when you have had him at your mercy for five years?"

None of these questions were "Why did you leak secrets to Dumbledore?" If Voldemort didn't tell Bellatrix, his most trusted advisor, there is a large chance he didn't know. For a full account of Snape's answers to Bellatrix, see Hogwarts is Here.

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