Modern international law is an all encompassing term that is uses to describe the law governing the relations between different countries (states). Hugo de Grotius is usually called the founder of international law because he is the first to publicize his treaty on the law of the sea. Grotius advocated the new principle in his 1609 writing "the free sea" that the sea was international territory and therefore all nations were free to use the sea for commerce. He wrote it in a time period where different countries were claiming the right to certain bodies of water and passageways which had given way to international disputes among nations competing for resources and discovery in the new world. One blatant example was the privateering that was used by Elizabethan England to plunder Spanish ships and using the justification that they were in English territory. Countries came to base their maritime claims on the principle that the maritime possessions extended seawards from land which then "maritime dominion to the actual distance within which cannon range could effectively protect it." This became universally adopted and developed into the three-mile limit.
This is also why piracy is still a crime that is governed by international law and also where the term "international waters" comes from. This is the first example of the accepted principle of the law of the sea being used for disputes for international law.