Hugo Grotius has been called by many, the father of international law. His treatise on "the Law of War and Peace; De Jure Belli ac Pacis" is said to contain a general theory of international law. This paper aims at reviewing Grotius' ideas on the relationship between natural law and positive law, war and ethics, as well as human will and natural justice. His perspectives in both "jus ad bellum" and "jus in bello" may at first sight seem obsolete, with no practical uses at the present time. However, the fact is that they have had a major role in the evolution of the philosophy of law. Furthermore, with the increasing relevance of human rights in international law, particularly in the last half a century, natural law and ethics are gaining a new stance in the system. Studying Grotius' ideas in the context of the circumstances prevailing in his time, we can dismiss much of the criticism made against what he rendered lawful belligerent rights, through the concept of "permission". Taking this approach, we can even track down the early traces of "universal jurisdiction" in his writings.