This title appears to be self-contradictory. How can there be a law of war if war is outlawed? This apparent contradiction disappears, howover, if the distinction between “ war in the legal sense” and “ war in the material sense” is kept in mind. War in the legal sense has been in large measure “outlawed” ; that is, the international law conventionally accepted by most states no longer recognizes that large-scale hostilities may constitute a “state of war” in which the belligerents are legally equal. Such hostilities may, however, occur constituting war in the material sense, and “laws of war” may regulate such activities. This has, in fact, long been the situation within the state. All states “outlaw” domestic insurrection, rebellion and civil strife, yet if these events occur, they apply the laws of war to the hostilities involved in their efforts at suppression. The first modern codification of the law of war was, in fact, issued by the United States to apply to civil strife. Lieber's Code, however, distinguished the applicability of the rules of land warfare in international war and in civil war:
When humanity induces the adoption of the rules of regular war toward rebels, whether the adoption is partial or entire, it does in no way whatever, imply a partial or complete acknowledgment of their government, if they have set up one, or of them, as an independent and sovereign power…. Nor does the adoption of the rules of war toward rebels imply an engagement with them extending beyond the limits of these rules…. Treating, in the field, the rebellious enemy according to the law and usages of war has never prevented a legitimate government from trying the leaders of the rebellion or chief rebels for high treason, and from treating them accordingly, unless they are included in a general amnesty.