Customary international law of the sea has developed over centuries

The United Nations did not invent the law of the sea. There has been a law of the sea in effect for many centuries. When Spanish and Portuguese explorers first charted new sea routes to the Americas and Asia, their governments imagined that they could lay claim to all the ocean vastness in between. Successful challenges by new maritime powers, especially Britain and Holland, soon established the principle that the high seas should be open to all. In the early 17th century, the Dutch jurist Hugo Grotius published an extremely learned treatise which summed up the new approach in a catchy phrase: “freedom of the seas.” To secure freedom on the seas, there had to be rules applicable to most situations that also acknowledged—and thereby constrained—necessary exceptions. These rules were developed over centuries in a process of mutual accommodation—and occasional challenge by war at sea—among major maritime powers. Nearly all of this law was “customary law,” meaning that it reflected actual practice among maritime states—including particular agreements among particular states—without being set down in any formal document.