What law is international law?
A set of rules generally regarded and accepted as binding in relations between states and nations.
Also called law of nations.
Or suitable law of international society.
Birth of international law
ウェストファリア条約(英:Peace of Westphalia)or ヴェストファーレン条約(独:Westfälischer Friede)
The prevailing view in the study of international law is that it emerged in Europe in the period after the Peace of Westphalia(1648), which concluded the Thirty Years War.
In state practice, the year 1648 marking the Peace of Westphalia is considered as a watershed, at least in Europe where a new policital order was created, to be replaced only after the defeat of Napoleon by the Vienna Congress of 1815.
There are two key points to answer the question that why the modern birth of international society had begun with Peace of Westphalia.
First is clear recognizion of birth of sovereign state.
The sovereing state we know today is found in a fall of medieval feudal system in West Europe as absolute sovereign state.
Second is the completion of coexistence of sovereign state.
In 16th century at West Europe, England and France were about to establish absolute sovereing state.
The Peace of Westphalia determined teritories. The Peace of Westphalia virtually demolished the Holy Roman Empire. Regional lords had been recieved sovereingty and recognized as independent nation.
The Peace of Westphalia denied universal authority of the Holy Roman Empire and Pope and represented the establsihment of modern sovereing state mean.
This is the first time but the international society was found as coexistence of sovereign state at limited area West Europe.