Mare clausum

Mare clausum (legal Latin meaning "closed sea") is a term used in international law to mention a sea, ocean or other navigable body of water under the jurisdiction of a state that is closed or not accessible to other states. Mare clausum is an exception to mare liberum (Latin for "free sea"), meaning a sea that is open to navigation to ships of all nations. In the generally accepted principle of International waters, oceans, seas, and waters outside national jurisdiction are open to navigation by all and referred to as "high seas" or mare liberum. Portugal and Spain defended a Mare clausum policy during the age of discovery. This was soon challenged by other European nations.

From 30 BC to 117 AD the Roman Empire came to surround the Mediterranean by controlling most of its coasts. Romans started then to name this sea mare nostrum (Latin for "our sea"). At those times the period between November and March was considered the most dangerous for navigation, so it was declared "mare clausum" (closed sea), although bans on navigation were probably never enforced. In classical law the ocean was not territorial. However, since the Middle Ages maritime republics like the Republic of Genoa and the Republic of Venice claimed a "Mare clausum" policy in the Mediterranean. Also Nordic kingdoms and England, required passage rates, monopolies on fishing and blocked foreign ships in their neighboring seas.

During Age of discovery, between the 15th and 17th century, sailing that had been mostly coastal became oceanic. Thus, the main focus was on long-haul routes. Countries of the Iberian Peninsula were pioneers in this process, seeking exclusive property and exploration rights over lands discovered and to be discovered. Given the amount of new lands and the resulting influx of wealth, the Kingdom of Portugal and the united kingdoms of Castile and Aragon began to compete openly. To avoid hostilities, they resorted to secrecy and diplomacy, marked by the signing of the Treaty of Alcáçovas in 1479 and the Treaty of Tordesillas in 1494.

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