This an unparalleled book of intellectual, legal, and military history, one that shows how ideas can exert real power in shaping events, and how events can in turn shape ideas.
Throughout much of history, the "laws of war" was regarded as an oxymoron. Cicero said that "in times of war, laws are mute," while Cervantes agreed that "all was fair" in war (or love). New Enlightenment ideals, however, tried to cabin the horrors of war and make it more civilized. The philosopher and poet Emmerich Vattel argued that since war was between two nations not two peoples, women, children, the old, and prisoners of war should be untouched. The "justice" of each side should be ignored while the war was ongoing, so mercy and restraint could be exercised. The United States, more than any other country, was founded on these new ideals about the legal practice of war. Our Declaration of Independence largely rested on the idea that the British King was waging a war "unworthy [of] the Head of a civilized nation," by attacking civilians, "plundering" ships, and quartering troops in private homes. The new nation went even farther than the legal ideals of Vattel, however, and formed treaties across Europe that promised that in any war between the two, both private persons AND property would not be attacked (and "free ships" of neutral parties would be untouched). Witt points out though, that our peculiar insistence on leaving "property" untouched came from our fear of opponents liberating American slaves, such as Lord Dunmore had attempted in the Revolution.
The attempt to make warface civilized, however, was upset when opponents acted in an "uncivilized" manner. Legal theorists had long claimed that the laws of war had no force against "savages," so American reprisals against Indians were untouched by these ideas. When Mexican guerillas attacked General Winfield Scott's forces in the Mexican-American War in 1848 in a manner similar to the Indians, however, he did not know what to do with them. They were technically civilized combatants, but they acted too like spies or "savages" who could be executed. Scott thus formed the first "military commissions" to try these guerillas, and brought a sort of private justice and punishment to even captured prisoners the laws of war once seemed to forbid.
The Civil War, however, was the real crucible of American ideals. The war itself eventually became a war to free slaves, which meant of course taking property away from the U.S.'s Southern opponents. Francis Lieber, a German lawyer and law professor at Columbia, wrote the first comprehensive law of war code for the U.S. Army in 1863, and it was aimed largely at eradicating slavery. It sanctioned harsh measures and pleaded the rights of "military necessity" to attack civilians and civilian property when deemed essential for the war effort. As Lieber said in the code, "Sharp wars are quick wars." In this war then, when the U.S. felt itself on a moral crusade, it brought harsher forms of war back, even as it created the first legal code to control them. As Witt shows, General Sherman's March to the Sea was not a denial of the new laws of war, it was the culmination of this new code of necessity. In order to free the slaves, such contraventions of previous ideals seemed necessary, and perhaps were. Even at the time people knew there was no easy answer here.
The book continues to deal with debates on the next Indian Wars, and the "water cure" tortures of rebels in the Philippines war (in a shocking parallel to the modern day, there was a debate at the time about whether harsh interrogation practices helped get information that led to the capture of the rebel leader Emilio Aguinaldo). Lieber's Code was applied in these new wars, and was soon adopted in the Hague Conventions after 1899 and the future Geneva Conventions after World War II. This CIvil War code forms the basis of most of our modern laws of war. The book shows that legal theorists such as Lieber did shape how war was practiced throughout American history, even if personal and national interests always shaped their own views. It also shows that there is no simple answer to what is "legal" in a war, and that the debate about such practices must continue as long as war itself does.