The Stanford Encyclopedia of Philosophy: Jon Miller's "Hugo Grotius"
Read section 3, on natural law. Grotius has come to be considered the "father of natural law." Take note of how Grotius gives primacy to natural law over other conceptions of law as well as how he aligns natural law with public reason (and agreement) between nation-states.
Click http://plato.stanford.edu/entries/grotius/ link to open resource.