In the fateful closing days of 1862, just three weeks before Emancipation, Abraham Lincoln's top military advisors commissioned a code of rules to govern the armies of the United States in a newly intensified war effort. The code Lincoln issued the next spring helped shape the remaining two years of Civil War. Its rules on torture, prisoners of war, assassination, and more, quickly became foundations of the modern laws of war and today's Geneva Conventions. Yet the hidden story of Lincoln's code, and of the decades of controversy that lay behind it, has never been told. In this masterful and strikingly original history, John Witt charts the alternately troubled and triumphant course of the laws of war in America from the Founding Founders to the dawn of the modern era, revealing the history of a code that reshaped the laws of war the world over.