Thank you for letting me know this is Dame's Rocket, Dame's Violet, or Hesperis Matronalis.
It is definitely a self-naturalizer .. for a long time I was afraid to pull it because I thought it was something my mother had planted. I knew my mother had planted phlox and I thought that this might be phlox. I'm so grateful to now that it isn't. Now I know it's just a wildly successful plant.
Thank you Lyn Vosbury and Silversurfer.