As Wolfgang Friedmann explained in his classic 1964 work, The Changing Structure of international law, "The principal preoccupation of the classical international law, as formulated by Grotius and the other founders, was the formalization, and the establishment of generally acceptable rules of conduct in international diplomacy." Note that these were formal rules designed to facilitate informal interaction. They formalized the process of diplomacy, not the substance of international cooperation. This is, this international law simply formed the preconditions for informal and ad hoc diplomatic action, rather than the contractual structure of formal cooperation over international regulatory issues. This was the international law of coexistence. It also included the regulation of war. War was the first area in which cooperation became desirable