"Basically," Ron shrugged. "Harry told you the prophecy, but when Tom first heard about it, he didn't get more than the first two lines, >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 ...< yadda, yadda, yadda. There was the question of Harry or Neville being the intended target. When Peter turned traitor, Tom took it as a sign and went after the Potters. As we all know, that didn't work particularly well for him. When he and Harry met face to face ... well, body to body after the Triwizard Tournament, Harry learned that he had a choice. The choice to kill Voldemort or to work with him."