In May 1865, the final month of the Civil War, the U.S. Army arrested and prosecuted a sitting congressman in a military trial in the border state of Maryland, though the federal criminal courts in the state were functioning. Convicted of aiding and abetting paroled Confederate soldiers, Benjamin Gwinn Harris of Maryland's Fifth Congressional District was imprisoned and barred from holding public office. Harris was a firebrand--effectively a Confederate serving in Congress--and had long advocated the constitutionality of slavery and the right of states to secede from the Union. This...