In the 16th century Portugal claimed the Indian Ocean and Spain the Pacific Ocean as their unique domain, as “closed seas.” In 1609, a Dutch jurist presented a new alternative that has since entered international law: the freedom of the seas.
It’s difficult to own an ocean. You cannot build fortifications or fences on it, you cannot erect towers and castles to keep people out. Nevertheless, the concept of international waters is surprisingly modern, and it only emerged out of a century-long debate during the so-called “Age of Discovery.”
Portugal in the 16th century was an enormous naval power. Having broken the monopoly of the land-bound Silk Road, they grew rich bringing spices to Europe via the Indian Ocean (as I wrote about in Portugal vs. Egypt in India). Such was their presence in the Indian Ocean that the country made an extraordinary claim: the Indian Ocean was theirs exclusively. It was a “closed sea,” a mare clausum.
As you can imagine, most of the other European powers did not acknowledge this claim. They had their own designs on these strange new worlds, and various wars and conflicts tested Portugal’s right to a whole ocean.
Spain, another major naval power, did not object. The Spanish and Portuguese Empires, through a series of treaties, had negotiated a compromise over overseas territory. Portugal would get the Indian Ocean, but Spain would get the Pacific Ocean as its own mare clausum. Between them, the two powers could rule the waves:
Other naval powers were keen to test this claim. In 1603, off the coast of Singapore, three ships of the Dutch East India Company seized the Santa Catarina, a Portuguese ship laden with silk, porcelain, and musk. Now, the Dutch and the Portuguese were at war, but it was still a controversial seizure. A Dutch lawyer and jurist named Hugo Grotius was deployed to find a justification for the capture of the Santa Catarina.
In Mare Liberum, Grotius argued that the open sea should be free to all. Countries had the immutable right to travel to other countries for the purpose of trade. The idea of a closed sea infringed this fundamental right:
The sea is common to all, because it is so limitless that it cannot become a possession of any one, and because it is adapted for the use of all.
Mare Liberum
Grotius may not have known it at the time, but he was initiating an international legal argument that ended with the creation of international waters: a free sea for you and me, the end of exclusive claims to entire oceans.
The Generalist Academy 
