‘Harry ... I as good as killed them,’ he croaked. ‘I persuaded Lily and James to change to Peter at the last moment, persuaded them to use him as Secret-Keeper instead of me ... I’m to blame, I know it ...’
Harry Potter and the Prisoner of Azkaban

It wasn’t Sirius who betrayed the Potters to Voldemort, but he did persuade them to make Pettigrew their Secret-Keeper. It’s a choice that ended up tormenting him – so what made Sirius suggest it? We’ve had a bit of an investigation into why this all happened.

But first of all, remind us what a Secret-Keeper is again?

To be a Secret-Keeper meant using the Fidelius Charm – ‘the magical concealment of a secret inside a single, living soul’. Such a charm was used to hide Lily and James Potter from Voldemort, after the Dark wizard targeted the couple as his next victims. Using a Secret-Keeper meant the Potters had no need to change location when they were hiding, as long as the Secret-Keeper kept their silence. It should’ve been the perfect way of protecting them.

And why was Sirius initially picked?

There were a couple of candidates: Dumbledore himself offered, but James was adamant that his best mate Sirius was the one. He apparently told Dumbledore that Sirius would die rather than betray them.

So did Sirius persuade them otherwise?

If James was so certain, it must have been Sirius’s persuasion that changed his mind – and that’s where the timing became important. Because as they were making these plans, everyone – Dumbledore, Sirius, probably the Potters themselves – suspected someone was sharing information with Voldemort. And while it’s strange that Pettigrew avoided suspicion, it seemed Sirius, at least, believed Lupin was the spy, hence why Lupin wasn’t told about the Secret-Keeper switch. It makes sense: if Sirius and James thought Lupin was passing information to Voldemort, then changing to Pettigrew while maintaining the illusion that Sirius was Secret Keeper would have kept him off the scent. Unfortunately, they were wrong about Lupin.

But then there was the ‘bluff’

As Sirius told Pettigrew: ‘I thought it was the perfect plan ... a bluff … Voldemort would be sure to come after me, would never dream they’d use a weak, talentless thing like you …’

Sirius was probably known to Voldemort for his family and for his closeness with the Potters. He was also a skilled member of the Order of the Phoenix and a particular enemy of Voldemort’s then-loyal follower, Snape. It doesn’t seem far-fetched to think Voldemort would have gone after Sirius, and he’d certainly have had no qualms about using Cruciatus or Imperius to force the secret out of him. Still, a Secret-Keeper can only break the Fidelius Charm voluntarily. Nobody doubts Sirius would, indeed, have died for his friends – so why not be Secret-Keeper anyway, given nobody could have tortured that secret out of him?

The answer is that Sirius wanted to deflect attention from the real Secret-Keeper. He probably hoped Voldemort would go after him so he could steer attention away from Pettigrew. If all had gone to plan, Voldemort may never have known Sirius was not the Secret-Keeper – but even if he’d discovered the bluff, Voldemort’s pursuit of Sirius would have given the Potters and Pettigrew time to regroup.

A crucial mistake

So far, so noble. Unfortunately for Sirius, everything he planned was based on an assumption he really should have known better than to make: the fact that Pettigrew was loyal. That, obviously, was wrong.

From what we know of the Marauders, Pettigrew was in awe of his clever friends. He needed James and Sirius’s help to turn into an Animagus and he didn’t have their confidence – a confidence that often spilled into arrogance. Perhaps Sirius assumed Pettigrew held them in such high regard he’d never betray them. That doesn’t mean he didn’t suspect Pettigrew would reveal their secrets under threat of torture, but he probably hoped his bluff would prevent Voldemort paying him any attention in the first place. He definitely didn’t expect Pettigrew to seek Voldemort out.

Overestimating Peter Pettigrew

Sirius overestimated Pettigrew’s loyalty because he would rather die than betray his friends, but he underestimated Pettigrew’s character in part because of his arrogance. Only a thoughtlessly clever, good-looking, confident man could fail to see how those traits might inspire resentment, and Sirius could be thoughtless. He admitted it himself: he should have known Pettigrew was the spy. Pettigrew was always ‘sneak[ing] around people who were stronger and more powerful…’ and Voldemort was the most powerful wizard of all. What was Pettigrew’s friendship with James, compared to that? He wasn’t his closest confidante, he wasn’t his best friend. So maybe when Sirius presented Pettigrew as an alternative Secret-Keeper, it was another example of his status as second-best.

Sirius spent his childhood as the odd one out; you’d think he might have picked up on a similar dynamic in his friendship group. But he seemed to have been insulated by being part of the Marauders, which made him blind to certain things. That he suspected Lupin over Pettigrew makes this clearer – he did not doubt the ‘weak, talentless’ Pettigrew would remain loyal, instead he suspected Lupin. Lupin would never have betrayed his friends either, but he was less rash than James and Sirius. James, Lupin told Harry, ‘would have regarded it as the height of dishonour to mistrust his friends’, but Lupin was always more cautious. Maybe it was this that made Sirius suspect him.

Of Pettigrew, he seemed to have barely given a thought.