There are several theories when the birth date of international law was. Hugo Grotius (1583-1645) was the first who systematized these specific rules and raised it to a scientific level. In this essay I examine how Grotius thought about important institutions of international law and what kind of impact these considerations have to our modern age. War cannot be seen only as an unlawful act, because most of the original human instincts can be recognized in fighting to each other. This point of view proclaims that international law does not denounce war generally. Existence of international law is important to determine the rules of warfare. To suit to the criterions of lawful war, a war should perform two requirements: opposite parties have ...