Lieber Code, promulgated by the Union during the American Civil War, was critical in the development of the laws of land warfare.

Historian Geoffrey Best called the period from 1856 to 1909 the law of war's "epoch of highest repute." The defining aspect of this period was the establishment, by states, of a positive legal or legislative foundation (i.e., written) superseding a regime based primarily on religion, chivalry, and customs. It is during this "modern"