Why did Grotius believe natural law could limit the power of sovereign states?

Hugo Grotius, often called the "father of international law," believed that natural law could limit the power of sovereign states because he viewed it as a system of justice inherent in human reason, which remains immutable even against the will of rulers.12

Writing during the violent Thirty Years’ War, Grotius sought to minimize bloodshed by establishing a framework of law that applied to all independent powers. He argued that natural law is derived from man’s inherent nature and possesses a degree of validity independent of religious belief, famously stating it would hold true even if one were to concede that God does not exist.13

By positioning natural law as a set of imperative considerations regarding justice and moral responsibility, Grotius provided a necessary check against the arbitrary exercise of political power. He maintained that because there is no supreme judge between nations, these rational, natural principles must serve to regulate and restrain the conduct of states, particularly regarding the resort to war and the violence committed within it.21