One of the questions I see over and over again in my travels around the internets is When Did Peter Defect (I thought I had written a post about all the questions I had about the Marauder’s generation, but I guess not maybe?) To Voldemort. It’s a pretty big deal – after all, it is the single catalyst for literally every single event in all seven books. Peter doesn’t defect, he doesn’t betray the Potters, the Potters don’t die, Voldemort doesn’t lose his power and doesn’t spend the next seventeen years trying to regain his powers and murder the shit out of Harry (granted then the First Wizarding War doesn’t end, but that’s a whole ‘nother plot). Why I didn’t realize is that the question has an answer.
Sirius says (PoA p374) that Peter had been passing information for a year prior to James & Lily’s deaths. I mean, I can’t really speak to how good of an answer that is, since it raises too many additional questions – how does Sirius know this? Are his sources reliable? What caused him to defect, and how did it go down? Did he start small and build up to betraying his best friends, or did he throw over his allegiance immediately? Did he ever regret his decision? But it’s the best answer we have right now. UNTIL JK GETS HER LIFE TOGETHER AND WRITE A MARAUDERS SERIES LIKE THE WORLD IS CRYING OUT FOR.