President Abraham Lincoln’s assassination on 14 April 1865 occurred with the assailant’s vow, sic semper tyrannus! (thus always to tyrants). Facing the imminent defeat of the secessionist cause, Confederates engaged in last-ditch conspiracies, including this murderous plot. National authorities hunted down and killed the trigger man, John Wilkes Booth, and they also captured eight of his alleged accomplices.

The new President, Andrew Johnson, faced a dilemma: should the state respect the defendants’ constitutional right to be tried in civil court? Or might a military commission try the suspects? In military trials, nine officers (rather than 12 jurymen) sat in judgement, and defendants could be convicted on the vote of a bare majority. The death penalty could be imposed if just six officers returned a guilty verdict. Johnson opted for a court martial, but he needed a gloss of legality to back his decision.

Attorney General James Speed obliged his President with a justification for swift and severe justice. Although General Robert E. Lee surrendered his Army of Northern Virginia at the Appomattox Courthouse on 9 April, the Union and the Confederacy remained at war on several fronts on the date of Lincoln’s death. Consequently, the suspects were not “citizens” but enemy spies. “To act as a spy is an offense against the laws of war,” Speed stated, “and the punishment for which in all ages has been death.”

The trials of the seven men and one woman proceeded over the spring of 1865 and each was convicted and sentenced to death for “maliciously, unlawfully, and traitorously” conspiring to kill the President. However, the verdict posed a second dilemma: should the eight convicts face execution, or should Johnson exercise his prerogative to pardon?

During the war Lincoln had pardoned rebellious citizens as an inducement to loyalty, and Johnson was equally appreciative of mercy’s capacity to secure the shaky peace. On 5 July 1965 he reached a compromise by ordering the execution of four convicts (including Mary Surratt, the first woman executed by the U.S. government) and commuting the death sentences of the other prisoners. Just prior to leaving office in March 1869, President Johnson pardoned the three remaining conspirators (one having already died).

Over Johnson’s controversial presidency he confirmed his Southern roots were far stronger than his belief in Reconstruction. Using his constitutional authority, the President granted mercy to thousands of rebels, using mass pardons as a tool of appeasement. The mercy Johnson bestowed on the conspirators in the Lincoln assassination played a small but symbolic role in affirming that racial equality would remain an unfulfilled ideal, not a national commitment.

Carolyn Strange specialises in criminal justice history. In semester 2 2015 she will teach Crime and Justice: Historical Dilemmas. She is currently completing a book on the history of pardoning and parole in New York State, from the Revolution to the Depression, to be published by New York University Press.