From space, the Earth shows coastlines, rivers and mountains but no national borders. Borders are human creations, yet people have fought and died over them for centuries, and a line on a map can decide which language a child learns at school, which laws protect her and which passport she carries.

State, nation and nation-state

These three terms are often confused. A state is a political unit with a permanent population, a defined territory and a government that claims sovereignty over it. A nation is a group of people who see themselves as sharing a culture, history and identity, whether or not they have a state of their own. A nation-state is a state whose borders roughly match those of a single nation; Iceland and Japan come close. Many states are multinational, such as Switzerland or India, and many nations are stateless or divided among several states. The Kurds, numbering perhaps 30 to 40 million, live mainly across Turkey, Iran, Iraq and Syria.

The modern idea of territorial sovereignty is often traced to the Peace of Westphalia in 1648, which ended the Thirty Years' War and helped establish that each ruler governed within fixed borders, free from outside interference. Historians now call this story simplified, but the principle spread worldwide. Territoriality is the attempt to control people and things by controlling a defined area. Landlords and street gangs practice it too.

Kinds of boundaries

Geographers classify boundaries in two main ways. By their form, some follow physical features such as rivers or mountain ridges, while others are geometric, following straight lines of latitude or longitude, like much of the border between the United States and Canada along the 49th parallel.

By their origin, the American geographer Richard Hartshorne set out a widely used scheme in the 1930s. An antecedent boundary was drawn before the area was heavily settled. A subsequent boundary was drawn after settlement, often following cultural divides, such as the 1947 partition line between India and Pakistan, which tried and largely failed to separate Hindu-majority and Muslim-majority areas, and set off one of the largest and bloodiest migrations in history. A superimposed boundary is one forced on an area by powerful outside states, with little regard for the people living there. A relic boundary no longer functions as a border but still leaves marks on the landscape, like the line of the Berlin Wall or the old border between East and West Germany, which still shows up in maps of wages and voting.

Colonial lines

Superimposed boundaries are a lasting legacy of empire. At the Berlin Conference of 1884 to 1885, European powers set rules for claiming African territory, and over the following decades they drew most of the continent's borders, splitting some peoples and grouping rivals together. The secret Sykes-Picot agreement of 1916 between Britain and France divided much of the Ottoman Middle East into zones of control and influence. How much these lines explain later conflicts is debated. Some scholars argue that arbitrary borders lie behind many wars and weak states. Others note that newly independent African states agreed in 1964, through the Organization of African Unity, to keep the colonial borders, precisely because redrawing them could have invited endless wars, and that most African borders have since proved remarkably stable.

Borders at sea

Territory does not stop at the shore. Under the United Nations Convention on the Law of the Sea, agreed in 1982, a coastal state has a territorial sea of 12 nautical miles and an exclusive economic zone stretching about 200 nautical miles from its coast, where it controls fishing, oil, gas and minerals. A single small island can therefore generate a huge zone of ocean, which is one reason states quarrel bitterly over rocks and reefs in the South China Sea.

An enclave is territory entirely surrounded by another state, like Lesotho inside South Africa.

Worked example: the Cooch Behar enclaves

Until 2015 the border between India and Bangladesh contained the strangest patchwork on Earth: 111 Indian enclaves inside Bangladesh and 51 Bangladeshi enclaves inside India. Some were enclaves within enclaves, and one, Dahala Khagrabari, was a patch of India inside a patch of Bangladesh inside a patch of India inside Bangladesh. The pattern is usually traced to scattered landholdings of local rulers centuries earlier, which were frozen into an international border when British India was partitioned in 1947 and East Pakistan later became Bangladesh.

For decades roughly 50,000 residents lived in a kind of practical statelessness. To reach a school, hospital, police station or court run by their own country, they had to cross foreign territory, often without papers. In 2015 the two countries finally swapped the enclaves, and residents were allowed to choose their citizenship. Most chose the nationality of the country that surrounded them, and many gained access to electricity, schools and identity documents for the first time. The case shows that a border is more than a line: being on the wrong side of one can strip a person of the practical rights that citizenship is supposed to guarantee.