How Are National Borders Drawn? Rivers, Rulers and Old Treaties

Political geographers sort boundaries by what determined them, and the categories predict how much trouble each causes:

• •Physical boundaries follow a natural feature, usually a river, a watershed or a mountain ridge, which is convenient and creates its own problems when the feature moves
• •Ethnographic or cultural boundaries follow the distribution of a language or people, which minimises grievance and is impossible to draw cleanly anywhere populations are mixed
• •Geometric boundaries follow lines of latitude and longitude, quick to specify and indifferent to everything on the ground
• •Antecedent boundaries were drawn before an area was substantially settled, so the population grew around them, as much of the American and Canadian west was
• •Subsequent boundaries were drawn after settlement and had to accommodate what was already there
• •Superimposed boundaries were forced on an existing population from outside, which describes most colonial borders
• •Relict boundaries no longer have legal force and remain visible, like the line between the two Germanies, which is still detectable in voting patterns and infrastructure

A river looks like an ideal boundary and is a poor one, because rivers move. The law distinguishes two processes, and the distinction has decided real cases: accretion, the gradual shifting of a bank by deposition, generally moves the boundary with it, while avulsion, a sudden change of course in a flood, generally leaves the boundary where the old channel was. The Rio Grande has produced a century of disputes on exactly this basis, including the Chamizal tract at El Paso, which changed hands after the river shifted and took until 1963 and a presidential agreement to settle. Rivers also fail as boundaries for a social reason, since a river valley is usually a single community with the same language and economy on both banks, so a border along it cuts through a unit rather than between two. The stronger natural boundary is a watershed, which genuinely separates catchments and populations, which is why the Andes and the Pyrenees work better as borders than the Rhine or the Mekong.

The African case is the clearest. The Berlin Conference of 1884 and 1885 set rules for European claims on the continent and was attended by no African representatives, and the boundaries that followed were agreed in European capitals, frequently by officials working from incomplete maps. The results split several hundred ethnic groups across borders, forced unrelated groups into single states, and created sixteen landlocked countries. Similar exercises produced the borders of the Middle East, where the Sykes-Picot agreement of 1916 divided Ottoman territory between British and French spheres, and the partition of India in 1947, where Cyril Radcliffe was given about five weeks to draw a line through Punjab and Bengal, had never visited India before, and worked largely from outdated census data; the resulting migration and violence killed somewhere between several hundred thousand and two million people. The striking fact about these lines is that they have almost all survived, because the African Union's predecessor adopted a principle in 1964 of respecting inherited borders, on the reasoning that opening any of them would open all of them.

The process has three technical stages that are frequently confused. Allocation is the political decision about which state gets what territory. Delimitation is writing the boundary down, in a treaty text and on a map, and this is where ambiguity enters, since a phrase such as the crest of the mountains or the main channel of the river has to be interpreted later. Demarcation is putting physical markers on the ground, which is expensive, and a very large proportion of the world's boundaries have never been demarcated at all. Disputes usually arise from the gap between stages: a treaty says one thing, a map attached to it shows another, and the ground matches neither. International tribunals resolve these by examining the treaty text, the maps, the conduct of the parties since, and the principle inherited from Roman law that a state succeeding a colonial administration inherits its administrative boundaries, which is how several African and Latin American cases have been decided.

Roughly a hundred land and maritime boundary disputes are active at any time, and several have specific well-documented causes. The line between India and China in the Himalayas rests on a boundary drawn at a 1914 conference that China did not ratify, and the two countries fought a war over it in 1962 and still hold periodic confrontations. Maritime boundaries have become more contested than land ones since the Law of the Sea Convention created two-hundred-mile exclusive economic zones, which turned isolated rocks into valuable assets because possessing one generates a zone around it, and which is the underlying driver of disputes in the South China Sea and the Aegean. Melting ice is producing new disputes in the Arctic, and it is also erasing some borders, since several alpine boundaries between Italy, Switzerland and Austria follow watersheds on glaciers that are retreating, forcing the states involved to adopt a moving boundary defined by whatever the ice does.

Some of the strangest borders are the ones that leave pieces of one country stranded inside another. The Belgian town of Baarle-Hertog consists of a couple of dozen separate fragments scattered inside the Dutch town of Baarle-Nassau, some of which contain small Dutch fragments of their own, the result of medieval land deals between local lords that were later frozen into national boundaries. The line runs through shops, gardens and houses, and a household's country has traditionally been decided by where its front door stands, which is why the border is marked on the pavement for everyone to see.

A more serious case sat on the border between India and Bangladesh, where about a hundred and sixty enclaves had survived from old arrangements between a local princely state and its neighbours. They included the world's only third-order enclave: a field belonging to India, inside a patch of Bangladesh, inside a patch of India, all surrounded by Bangladesh. Tens of thousands of people lived in these pockets with little access to schools, police or courts, since reaching their own state meant crossing a foreign one. The two governments exchanged them in 2015 and let residents choose their citizenship, showing that a border can be redrawn peacefully when both sides gain.

A border defined by a line of latitude sounds simple until someone has to find it on the ground. Latitude is measured from the sky, and before satellite positioning, survey teams fixed their position by timing stars through telescopes, then ran lines between those points through forest, swamp and mountain. Small errors in the observations, and the slight pull of nearby mountains on the plumb lines used to level instruments, meant that markers ended up a little off the intended line. The Mason-Dixon line, surveyed in the 1760s to settle a quarrel between two colonial proprietors, was a celebrated feat of this kind of work.

The border between Canada and the United States along the 49th parallel shows what happens next. The markers placed by nineteenth-century survey teams wander slightly north and south of the true parallel, and rather than move them, the two countries agreed by treaty that the boundary is the series of straight lines joining the monuments, not the parallel itself. The legal border is therefore what the surveyors marked, errors and all. A strip about six metres wide is still kept clear of trees along much of its length so the line stays visible. Once both sides accept it, a mistake on the ground becomes the law.

Countries have borders at sea as well as on land, and they are drawn by measuring outwards from the coast. Under the United Nations Convention on the Law of the Sea, agreed in 1982, a coastal state has territorial waters extending 12 nautical miles from its baselines, where it has nearly full control, much like its land territory.

Beyond that it can claim an exclusive economic zone stretching up to 200 nautical miles, giving it rights over fishing, oil, gas and minerals, though other countries' ships may pass freely. Where coasts are closer than 400 nautical miles apart, neighbours must agree on a dividing line, and disputes over small islands can matter enormously because each island can generate a large zone of sea around it.

Boundaries are classified by what determined them, and geometric lines drawn from a distance cause far more lasting trouble than ones following watersheds or settlement patterns, which is why a third of Africa's borders are straight and Europe's are not. Rivers make poor boundaries because they move and because a valley is usually one community. A border passes through allocation, delimitation in a treaty and demarcation on the ground, and most disputes come from gaps between those stages. Maritime boundaries are now the most contested, since two-hundred-mile zones made isolated rocks valuable.