On a sweltering July 7, 1865, a mere 12 weeks after Abraham Lincoln was shot at Ford’s Theater, four of his assassin’s accomplices were hanged in the courtyard of the District of Columbia’s Washington Arsenal — present-day Fort McNair, and specifically its tennis courts. The exact nature of the conspiracy against the man who had seen the North to victory in the Civil War has been debated ever since actor John Wilkes Booth lodged a ball from his one-shot Derringer behind Honest Abe’s ear. But it was a conspiracy — an astoundingly bold one. Simultaneous with Booth’s successful attack upon...