Cooperation under anarchy when persons are from different social groups is one thing. When those groups are avowed enemies, it's quite another. The idea that self-governance could promote cooperation between socially distant hostiles seems absurd.

Yet it can, and it has. In the words of John Stuart Mill (1848: 882), “Insecurity paralyzes only when it is such in nature and in degree that no energy of which mankind in general are capable affords any tolerable means of self-protection.” That energy, you will see, is substantial.

This essay examines a significant and long-lasting era of intergroup anarchy among English and Scottish citizens on the Anglo-Scottish border in the sixteenth century. The border people pillaged, plundered, and raided one another as a way of life they called “reiving.” To regulate this system of intergroup banditry and prevent it from degenerating into chaos, border inhabitants developed a self-governing system of cross-border criminal law called the Leges Marchiarum. These “laws of lawlessness” governed all aspects of cross-border interaction and spawned novel institutions of their enforcement, including “days of truce,” bonds, “bawling,” and “trod.”