Formation of the state
The state was first advocated by Plato, then found more acceptance in the consolidation of power under the Roman Catholic Church. European monarchs then gained power as the Catholic Church was stripped of temporal power and was replaced by the divine right of kings. In 1648, the powers of Europe signed the Treaty of Westphalia which ended the religious violence for purely political governance and outlook, signifying the birth of the modern 'state'.
Within this statist paradigm, only the state and its appointed representatives were allowed to bear arms and enter into war. In fact, war was only understood as a conflict between sovereign states. Kings strengthened this idea and gave it the force of law. Whereas previously any noble could start a war, the monarchs of Europe of necessity consolidated military power in response to the Napoleonic war.