Maritime borders are the lines that divide the sea between states, measured outward from a coastal baseline under the 1982 United Nations Convention on the Law of the Sea (UNCLOS). A state has full sovereignty over its territorial sea out to 12 nautical miles (22 kilometres), limited policing rights in a contiguous zone to 24 nautical miles, sole rights to fish, oil and gas in an exclusive economic zone (EEZ) out to 200 nautical miles (370 kilometres), and rights over the seabed of its continental shelf, which in places can reach 350 nautical miles. Where neighbours' zones overlap, the line between them is agreed by treaty or drawn by a court, usually starting from a line equidistant from both coasts. Those rules were not designed in one sitting; they were argued, fought and litigated into shape over four centuries.
1609: Grotius, Selden and the argument over a free sea
The modern story begins with a legal brief. In 1608 the Zeeland Chamber of the Dutch East India Company asked the young jurist Hugo Grotius to defend its seizure of the Portuguese carrack Santa Catarina. The result, published in 1609 as Mare Liberum ("The Free Sea"), argued that the ocean was international territory that no nation could own, and that the Dutch therefore had every right to sail to the East Indies and trade there. Its target was the Portuguese policy of mare clausum, the closed sea, under which Portugal claimed a monopoly over the East Indian trade.
The English lawyer John Selden answered in Mare Clausum (1635), arguing that the sea could be appropriated much like land. Neither book drew a line on a chart, but between them they set the terms of every later dispute: how much of the sea a coastal state may own, and where the free high seas begin. As claims multiplied, maritime states moderated their demands and settled on the idea that ownership should extend only a limited distance from the shore.
1702: Bynkershoek's cannon shot and the three-mile belt
That distance was supplied by the Dutch judge Cornelis van Bynkershoek. In De dominio maris (1702) he argued that a state's power over the sea ends where the power of its weapons ends, so its waters should reach as far as a cannon on shore could defend. The Italian writer Ferdinando Galiani later put the range of the best cannon of his day at three nautical miles, about one league, and the "cannon shot rule" hardened into the three-mile limit.
From the 18th century until the mid-20th, the British Empire, the United States, France and many other states claimed three miles. The system was never uniform: Iceland claimed two nautical miles, Norway and Sweden four, and Spain six. A League of Nations conference at The Hague in 1930 tried to agree a single width and failed. Three miles survives today only in pockets; as of 2011 Jordan was the only state still using it for its whole coast, and the United Kingdom itself moved to twelve with the Territorial Sea Act 1987.
28 September 1945: the Truman Proclamation and the 200-mile claims
The cannon-shot era ended over oil and fish. On 28 September 1945 President Harry S. Truman proclaimed United States control over the natural resources of its continental shelf, the shallow seabed extending beyond the three-mile limit. It was one of the first claims to exclusive jurisdiction beyond territorial waters, and others followed quickly.
The most far-reaching came from South America. On 23 June 1947 Chile's President Gabriel González Videla claimed a maritime zone of 200 nautical miles, and on 1 August 1947 Peru's President José Luis Bustamante y Rivero did the same by decree. Ecuador followed. Their aim was to protect the rich fishing grounds of the Humboldt Current, and 200 nautical miles, far wider than any cannon could reach, would eventually become the global standard for economic rights at sea. By 1967, a count of national claims showed how fragmented the map had become.
1958–1976: the Geneva conventions and the Cod Wars
The United Nations tried to codify the new order at Geneva. The first Conference on the Law of the Sea in 1958 produced four treaties, on the territorial sea and contiguous zone, the continental shelf, the high seas and fisheries, which entered into force between 1962 and 1966. They left open the most important question, the width of the territorial sea, and a second six-week conference in 1960 failed to settle it.
Iceland settled it for itself. It widened its limit from three to four nautical miles in 1952 and to twelve in 1958, banning foreign fleets. Britain refused to accept this, and three confrontations followed, in 1958–61, 1972–73 over a 50-mile fishery limit, and 1975–76 over a 200-mile one. The Royal Navy escorted British trawlers while the Icelandic Coast Guard cut their nets with hawsers, and ships on both sides were damaged by ramming. One Icelandic engineer died, killed accidentally in 1973 while repairing damage to his ship. In a NATO-brokered agreement in 1976, Britain accepted Iceland's 200-mile fishery zone, ending more than 500 years of unrestricted British fishing off Iceland; Iceland had won every round.
1969: the North Sea Continental Shelf cases and the median line
Once states owned the seabed, they needed a way to divide it where coasts were close. The 1958 Continental Shelf Convention offered one in Article 6: absent agreement or special circumstances, the boundary should be the median line, every point of which is equidistant from the nearest points of each coast. In the North Sea that rule hurt West Germany, whose coast curves inward between the outward-bulging coasts of the Netherlands and Denmark, so equidistance lines drawn from both neighbours would have left it a disproportionately small share of the oil- and gas-rich shelf.
Germany had not ratified the convention, and in 1969 the International Court of Justice agreed that equidistance was not a binding customary rule. It told the parties to negotiate so as to abate the effects of an incidental feature, Germany's concave coast, that would otherwise produce an unjustifiable difference in treatment. In the talks that followed, Germany obtained most of the extra shelf it had sought. The case made "equitable result" the goal of maritime delimitation, and tribunals have applied that idea since, as in the 1992 Canada–France arbitration over Saint Pierre and Miquelon, which awarded France 18 percent of the area it claimed.
10 December 1982: the Law of the Sea Convention
The third UN conference opened in New York in 1973, prompted by a 1967 speech at the UN by Malta's Arvid Pardo, and ran for nine years with more than 160 nations negotiating by consensus. The convention was opened for signature on 10 December 1982 and entered into force on 16 November 1994, a year after Guyana became the 60th state to ratify. As of October 2024 it had 169 state parties plus the European Union; the United States has not ratified it.
UNCLOS fixed the zones measured from the baseline, normally the low-water line: internal waters, a 12-mile territorial sea with a right of innocent passage for foreign ships, a 24-mile contiguous zone for customs, tax, immigration and pollution enforcement, and a 200-mile EEZ. It let archipelagic states draw baselines around their outer islands, and allowed an extended continental shelf beyond 200 miles, but never more than 350 nautical miles from the baseline or 100 nautical miles beyond the 2,500-metre depth line. The Iceland country guide shows what a 200-mile zone means to a nation that lives from its fishing grounds.
2009: Snake Island and the Black Sea judgment
With the zones fixed, the fights moved to how islands count. Romania and Ukraine had agreed in 1997 that if they could not settle their Black Sea boundary within two years, either could go to the International Court of Justice. Romania filed on 16 September 2004. Oil and gas had been found under the disputed seabed, and the key question was Snake Island, a small rocky outcrop off the Danube delta. If it counted as a full island it would generate a large zone for Ukraine; if not, far less. In February 2007 Ukraine founded a village, Bile, on the island to strengthen its case.
The court ruled on 3 February 2009. It held that counting the island as part of Ukraine's coast would amount to "a judicial refashioning of geography", and gave it no effect on the boundary beyond the 12-mile arc of its own territorial sea. Romania received nearly 80 percent of the disputed area.
12 July 2016: the South China Sea award
The same questions, rock or island and history or convention, came to a head in the South China Sea. After a 2012 standoff at Scarborough Shoal, a reef about 220 kilometres from Luzon, the Philippines began arbitration against China under UNCLOS in 2013. China declared on 19 February 2013 that it would not take part and has argued that the tribunal lacked jurisdiction.
On 12 July 2016 the tribunal ruled that China's claims to historic rights over maritime areas within the nine-dash line had no lawful effect beyond what UNCLOS allows, and that Scarborough Shoal is a rock that generates no EEZ; it also classed Itu Aba, the largest Spratly feature, which Taiwan administers, as a rock. It expressly did not rule on sovereignty over any feature or draw any boundary. China and Taiwan both rejected the award, and the shoal has been under de facto Chinese control since 2012. Four hundred years after Grotius, the question of who may own the sea is still being argued feature by feature, as our guide to the South China Sea explains.