The May 4 federal seditious conspiracy convictions of four Proud Boys, who led the violent attempt to derail certification of Joe Biden's presidential victory on Jan. 6, 2021, mark an important milestone in the effort to protect our democracy from internal attack, as did the earlier convictions of six Oath Keepers and the guilty plea of another Proud Boys leader on the same charge. Political leaders should quickly leverage these triumphs of justice to purge members of such violent extremist groups from the nation's law enforcement ranks before illiberal winds slam the window of opportunity shut. My thriller, Shadow State,...