The modern concept of the state system, based on the principle of sovereignty, is widely traced back to the Peace of Westphalia in 1648.
This series of peace treaties ended the Thirty Years' War in the Holy Roman Empire and the Eighty Years' War between Spain and the Dutch Republic.
The treaties established the principle of state sovereignty, meaning that each state has exclusive sovereignty over its territory, is equal to other states, and foreign powers should not interfere in its domestic affairs.
This "Westphalian system" became the foundation of modern international relations.