Gary J. Bass, the author of “Stay the Hand of Vengeance” and “Freedom’s Battle,” is finishing a book about the Nixon administration and the 1971 massacres in Bangladesh.

In recent years, the American debate about war — about torture, drone strikes, the killing of civilians in Afghanistan and Pakistan — has also been a debate about the international laws of war. But much of this law, as John Fabian Witt shows in his magnificent new book, was originally made in America.

Abraham Lincoln’s administration published a new fighting code for Union soldiers in 1863, which diffused far beyond American shores: to the Prussian Army in 1870, into the landmark Hague Convention in 1899, and even into the Geneva Conventions and the Nuremberg trials after World War II. Witt, a professor at Yale Law School, writes that it was Francis ­Lieber, the Lincoln team’s foremost wartime legal authority, who — trying to figure out how Union troops should treat Southern irregulars — came up with some of the defining features of soldiers that guided the Third Geneva Convention in 1949: wearing distinctive insignia identifying them as combatants; operating under a command structure; and following the laws of war.

“Lincoln’s Code” is both a celebratory chronicle of American lawmaking and a gruesome record of American wartime cruelty, from William Tecumseh Sherman’s rampage through Georgia and South Carolina to the Indian wars. In an effort to make sense of what animates the “world’s only military superpower” today, Witt looks backward: “From the Revolution forward, the United States’ long history of leadership in creating the laws of war stands cheek by jowl with a destructive style of warfare.”...