How did Hugo Grotius use natural law to place moral limits on sovereign states?

Hugo Grotius, often called the "father of international law," utilized natural law to establish moral boundaries for sovereign states during an era of intense conflict, such as the Thirty Years’ War.12

In his seminal work, De Jure Belli ac Pacis (On the Law of War and Peace), Grotius argued that natural law is rooted in human reason and is immutable, meaning it remains valid regardless of the will of sovereign rulers or even the existence of a deity. By positioning natural law as a standard of justice inherent to human nature, he provided a framework to check the arbitrary exercise of political power.23

Grotius sought to minimize bloodshed by asserting that war should be regulated by a "common law" among nations. He argued that only wars with "just causes"—such as defense or the restitution of property—should be permitted. To enforce these limits, he employed a multilayered approach, supplementing secular natural law with religious norms and arguments of utility to restrain the conduct of states and discourage cruelty in warfare.2