Wednesday's shooting at the Holocaust Museum in D.C.—the second attack by a domestic terrorist in two weeks—killed one and left others injured. As details on the suspect, a white supremacist leader, come to light, a controversial government memo on right-wing terror earlier this year deserves another look. To the nation's horror, a much-maligned Department of Homeland Security memo on right-wing extremism is looking more accurate by the day. With news of a horrific incident of domestic terrorism Wednesday at the Holocaust Memorial Museum in Washington, D.C., the second major attack by a far-right gunman in less than two weeks, the...