While Bruce Wayne's traveling the world after Chill's trial, he runs into Lily (and James?) Potter, and saves her life.  She puts him in her will to take over guardianship of Harry if she and everybody else die, because he's away from the war and presumably safe from it.  But that's at the beginning of the 7 years he's gone, and obviously she dies before he returns, so the Dursleys get Harry until he returns and learns about it.