The law of war has played a central role in debates about American policies toward al-Qaida under Presidents Bush and Obama, with critics frequently arguing that the U.S. government has violated the law of war or improperly cited it as support for its policies. Witt’s historical account helps explain why both administrations felt it necessary to deviate from a strict interpretation of international law.
And here's a bit more on substance:
For many observers [of law of war violations during the Civil War], these examples admit of only one conclusion: that Lincoln chose to violate the laws of war when they stood in his way. He did not violate all the rules, but if one gets to pick and choose which rules to obey and which to violate, then the law lacks binding force. The law of war, which at the time was customary rather than embodied in treaties, was not worth the paper it wasn’t written on. Enthusiasts for the way that the Bush administration disregarded or rewrote the law of war to address the threat from Al Qaeda and the Taliban could cite none other than the sainted Lincoln as precedent.
Witt believes that this view is too simple. He agrees that Lincoln did not always follow the rules, yet he regards Lincoln as a pragmatist rather than as a criminal. What’s the difference? Witt points out that Lincoln did much to advance the law of war. In 1863, under Lincoln’s authority, a military theorist named Francis Lieber drafted a code of military conduct that became known among international lawyers as the Lieber Code, though Witt prefers to call it Lincoln’s Code, the title of his book. The Lieber Code codified large swaths of the unwritten customary laws of war and modified them where necessary to serve the Union’s immediate war aims—most notably in provisions that authorized belligerents to emancipate slaves owned by persons in enemy countries and that banned discrimination against soldiers on the basis of race. The code was also notable in its insistence that “military necessity” trumped (nearly) all of the substantive rules it contained. So although the Code humanely prohibited belligerents from executing POWs, it also grimly permitted them to do so in the service of “military necessity.” All that was ruled out was private violence or sadistic cruelty.\