 Garfield was one of three attorneys who argued for the petitioners in the landmark Supreme Court case Ex parte Milligan in 1866 . The petitioners were pro @-@ Confederate northern men who had been found guilty and sentenced to death by a military court for treasonous activities . The case turned on whether the defendants should instead have been tried by a civilian court , and resulted in a ruling that civilians could not be tried before military tribunals while the civil courts were operating . The oral argument was Garfield 's first court appearance . Jeremiah Black had taken him in as a junior partner a year before , and assigned the case to him in light of his highly regarded oratory skills . With the result , Garfield instantly achieved a reputation as a preeminent appellate lawyer . 
