Hugo Grotius was a Dutch jurist, philosopher, and theologian who is considered one of the founding figures of international law. He is best known for his work "De jure belli ac pacis" (On the Law of War and Peace), which laid the foundations for modern international law. Grotius also made significant contributions to the fields of natural law and political theory. He was a key figure in the development of the concept of freedom of the seas and the idea of just war. Grotius' ideas have had a lasting impact on the development of international relations and continue to be influential today.