I Histoneal background 1 Ever since the days of antiquity, Princes and States have concluded international treaties. In the vicissitudes of war and peace which form the fabric of history, even semantics seem to imply that the establishment of peace is linked to the conclusion of pacts. Yet, however interesting the fact may be that treaties between the Egyptian pharaoh and the Hittite king were concluded by an exchange of letters or that, closer to us, in the seventeenth century Grotius set out the principles of treaty interpretation in terms not unlike those of the 1969 Vienna Convention on the Law of Treaties, it is from 1815 onwards that the development of treaties has been especially remarkable: in a little over a century and a half, this essential instrument of international relations has undergone a tremendous transformation. Let us therefore begin by considering the underlying causes and main phases of this historical evolution.(*)

1. Underlying causes 2 The fundamental cause of the development has been the increasing solidarity between the components of international society: mechanical solidarity between States whereby any change in one of the components alters the balance of power within the whole system; solidarity of the general interests of mankind, requiring problems to be tackled in common and simultaneously; and solidarity between individuals in the development of culture and public opinion. 3 While these aspects of solidarity are first of all apparent

within the State, they also imply going beyond the confines of the State, and international treaties constitute the main legal mechanism which, although dependent on national institutions for purposes of conclusion and implementation, reaches further and transcends them. Any treaty could indeed be seen individually as a bridge boldly built out into the void of an international society which has yet to acquire real consistency; collectively the corpus of concluded treaties forms the reality and substance of an international society from which treaties in turn will increasingly derive their legal traits. In that process, nations starting out as entities closed to one another gradually open up and create the very environment to which they submit themselves.