How did Hugo Grotius use natural law to develop ideas about international law?

Hugo Grotius, often called the "father of international law," utilized natural law as the foundational framework for his masterpiece, De Jure Belli ac Pacis (1625).12

Driven by the violence of the Thirty Years’ War, Grotius sought to minimize bloodshed by establishing a system of jurisprudence that could regulate conflicts between independent powers. His approach included several key elements:2

• Secular Foundation: Grotius argued that natural law, derived from human nature and reason, possessed validity independent of religious belief. He famously asserted that these principles would hold true "even if we should concede... that there is no God".23
• Multilayered Norms: Recognizing that secular law alone might not restrain rulers, he integrated religious principles from the Old Testament and New Testament to reinforce his arguments.2
• Just War Theory: He posited that only wars with "just causes"—such as defense or the restitution of property—should be permitted, effectively using natural law to limit the scope of legitimate violence.2

By organizing these concepts into a comprehensive system, Grotius helped transform natural law from a theological concept into a secular philosophical tool for governing interstate relations.24