Who was Hugo Grotius (1583-1645) and what relationship does he have to the naturalist theory of international law?

Sometimes called the father of modern international law, Hugo Grotius was one of the pioneering natural rights theorists of the late 16th and early 17th centuries. He based his system on the law of nature and proposed the view that the already existing customs governing the relation between nations had the force of law and were binding unless contrary to natural justice. Grotius’s first published work on international law, Mare Liberum (The Free Sea, 1609), challenged the right of any nation to claim any part of the open sea as exclusively its own. Such a claim, Grotius argued, was against natural law and the basic law of humanity. He took the same line of argument in his work De Jure Belli ac Pacis (On the Law of War and Peace, 1625): that war violates natural law, which applies to the conduct of nations and of individuals. His contention was that war can be condoned only if it is for a righteous cause and conciliation has failed, and he called for humanitarian limits on such warfare.