Published in 2012, winner of the Bancroft Prize, I've finally gotten to it in my backlog.
The vast bulk of military history books consist consist of battle and campaign accounts, and biographies of military leaders. There are few books on more abstract subjects in the field. Mark Grimsley's classic The Hard Hand of War is a notable entry in the latter category. Grimsley covers the evolution of Federal military policies towards the Confederacy as they progressed during the war.
Lincoln's Code intersects with Grimsley's work in an interesting way. While Grimsley focuses on the narrow 5 year span of the Civil War, Witt's timeline runs from George Washington's early experiences as a Colonial officer in Virginia during the French-and-Indian wars through to the Philippine Insurrection. Witt's focus is on the evolving state of the laws of war, and in particular, he points to the development of the Lieber Code in 1862, issued to the Union forces as General Orders 100, as a critical pivot point.
The development of the Lieber Code is a key shared point of interest for both Grimsley and Witt, but Witt approaches it by showing its relationship to that which came before, and the manner in which it evolved from a US-specific set of orders into the basis for the international laws of war today, as embodied in the Hague and Geneva Conventions.
Witt does not shy away from atrocities. Atrocities are an important element in the evolution of the code and on more than one occasion the US has perhaps not really lived up to its own ideals. The Indian wars are particularly challenging; the Indian tribes did not really understand or follow the principles of "civilized nations", and so were treated as savages who did not deserve the protection of the same. But then, the Philippine insurgents regarded the laws of war as giving an advantage to their oppressors, and so chose not to pay any attention, and in response, the US ignored many of the strictures in their own code. For example, Leiber explicitly banned torture in his code, but in the Philippines US forces frequently applied the "water cure", which is essentially the same thing as water boarding (some american commanders were courtmartialed and slapped on the wrist, but on the whole the violations were not taken very seriously.)
There is a major emphasis on the changes that the Civil War worked on the attitudes of the US. Lincoln and Seward reversed traditional US positions on slavery and neutral shipping. Witt argues that Leiber's code was an essential companion to the Emancipation Proclamation, as the prior US position was that slaves must be left alone by armies during war (the British having offered to free slaves who escaped and came over during the Revolutionary War and the War of 1812.)
I have long been fascinated by the way that the US treated the south as a simple rebellion but at the same time frequently treated them like a nation which was at war with the US, extending them appropriate courtesies (like treating captives as POWs instead of traitors.) Witt speaks directly to this, making a strong argument that Lincoln and Seward realized it wasn't one thing or the other, that they could have it both ways if they played it right. The British need the closure of the southern ports to be a blockade rather than a simple port closure for reasons of diplomatic nicety, so they made it a blockade. Gideon Welles (secretary of the navy) never got this, his diaries are full of discussion of how confused Lincoln and Seward were. But they only seemed confused.
This book is long and meaty, and not a quick read. The core text is about 375 pages in trade paper. It is an excellent treatment of a rarely discussed subject.
