The Jan. 6 committee takes its place alongside the Watergate panel, the Truman committee and a few other congressional investigations that leave a profound historical mark. While unique, it’s a case study in how to conduct a serious investigation. The careful, exhaustive and private preparation of interviews and gathering evidence set the stage for the compelling public hearings — which left no doubt that the President of the United States attempted a coup to remain in power and sabotage democracy. The panel, led by Democratic chairman Bennie Thompson (D-Miss.) and Republican Liz Cheney (R-Wyo.), showed that the Jan. 6 deadly...