Nation-State System AND The Treaty of Westphalia:
The modern state system is a recent phenomenon. This concept was spread by the European through colonization. Nation-state systems can be found in different times of human history. It can be seen in the dust of time if you go back to Athen and Sparta city-states. It was in India, China, and Italy. But at that time they don't know about such thing as the Nation-state system. But the birth of this idea can be seen in the treaty of Westphalia of 1648. The way to the Nation-state system was paved by the breaking of the Holy Roman Empire and the feudal order, writing of thinkers like Machiavelli, Jean Bodin, Althusius, Grotius, and Hobbes, etc. They give the concept of territory, population, sovereignty, and law.
European powers ( House of Habsburg and House of Bourbon) fight for dominance with each other. For thirty years they fought resulting in 4.5 million to 8 million dead. The desire for peace was everywhere in Europe. The Treaty of Westphalia was signed in 1648 ending the war. With this treaty, they recognized and laid the foundation for the state system. With this, they recognized the rules and obligations of the state. The sovereignty and territorial integrity of a state was recognized. Later on, this becomes the basis for the modern state system.
Westphalia is in northwest Germany which gives its name to the treaty. After thirty years of war, the treaty gives independence to the Netherlands from Spain and Switzerland got from Austria. France acquired Alsace-Lorraine, Sweden gain territory and payment in cash. Brandenburg and Bavaria also made gains too. The German principalities secure their autonomy. The Roman catholic conquest of Europe vanished and Protestantism is to stay.
The emergence of the supra-state systems such as the European Union and African Union etc created a challenge for the modern state system. Their autonomy is the hands of policy makers of the supra-state systems.
These systems going in contrast to the Nation-state system. For economic and defense they give up their sovereign right given by the Nation-state system.