Hugo Grotius (also known as Huig de Groot or Hugo de Groot; 10 April 1583 – 28 August 1645) worked as a jurist in the Dutch Republic. With Francisco de Vitoria and Alberico Gentili he laid the foundations for international law, based on natural law. He was also a philosopher, theologian, Christian apologist, playwright, and poet.(Wikipedia)
Publications: On the Law of War and Peace(1625).
He argued that "To proceed the war and during the war, there must be common law that must be followed by the nations"