As I understand it, diplomacy was originally just a set of agreements between individual rulers. These agreements died with the individuals involved and would have to be renewed by their successors. But now diplomacy is conducted between states and is, I think, perpetual. When and how did this change in style of diplomacy change? Why are treaties written between, for example, the US government and the Mexican government and not between Barack Obama and Enrique Peña Nieto?
To illustrate what I'm thinking of, here are some example diplomatic documents.
The messages I sent to your father and the wishes he expressed to me will certainly be renewed between us. O King, I did not reject anything your father asked for, and your father never neglected none of the wishes I expressed, but granted me everything. Why have you, my brother, refused to send me what your father during his lifetime has sent me?
His Brittanic Majesty and the United States of America, being desirous, by a treaty of amity, commerce and navigation, to terminate their differences in such a manner, as, without reference to the merits of their respective complaints and pretensions...
The Government of Canada, the Government of the United Mexican States and the Government of the United States of America, resolved to:
SeeThe treaties of Osnabrück and Münster otherwise known as the Peace or Treaty of Westphalia, that ended the Thirty Years War, is pretty commonly held to be the first European treaty to recognize sovereignty of the nation-state rather than the sovereign itself. Henry Kissinger in his book: World Order: Reflections on the Character of Nations and the Course of History specifically calls this treaty out as the root of modern nation-state diplomacy. So there's my $.02 on the question. As to the how, that's a bit trickier. The Peace of Westphalia guaranteed against interference from other powers agents in the sovereign territory of another power. And made the offending power responsible for provocations of war made by its subjects. That sort of responsibility transcends the rule of a single sovereign. As well, since the documents themselves are often considered to be heavily influenced by Cardinal Marazin, Cardinal Richelieu's protégé and replacement, it had the backing and in theory will to be enforced by France. Which by the end of the Thirty Years War stood pre eminent on the continent. So the principles of sovereignty laid by these treaties became the norm afterwards