Mare Liberum (or The Freedom of the Seas) is a book in Latin on international law written by the Dutch jurist and philosopher Hugo Grotius. In Mare Liberum, first published in 1609, Grotius formulated the new principle that the sea was international territory and all nations were free to use it for seafaring trade. The disputation was directed towards the Portuguese Mare Clausum policy and their claim of monopoly on the East Indian Trade. England, competing fiercely with the Dutch for domination of world trade, opposed this idea and, in response, issued a Mare Clausum document (The Closed Sea).
Grotius argued that the sea was free to all and that nobody had the right to deny others access to it. He based his argument on what he called the “most specific and unimpeachable axiom of the Law of Nations, called a primary rule or first principle, the spirit of which is self-evident and immutable”, namely that: “Every nation is free to travel to every other nation, and to trade with it.” From this premise, Grotius argued that this self-evident and immutable right required
(1) a right of innocent passage over land, and
(2) a similar right of innocent passage at sea.
Mare Liberum is a part of his legal thesis issued under the name De Jure Praedae, On the Law of Prize and Booty.