Borders between countries are determined by a mix of natural geography, treaties negotiated between neighboring states, inherited colonial administrative lines, and international legal rules that govern water, seabed, and airspace. When two governments disagree about where a line belongs, they resolve it through negotiation, arbitration, or a ruling from the International Court of Justice. No single method accounts for every boundary on the map; most borders reflect several of these forces layered on top of each other over time.
Natural Features as Borders
Some of the oldest borders follow the land itself. Mountain ranges like the Pyrenees between France and Spain, or the Himalayas between India and China, work as boundaries because they are hard to cross and easy to identify. Rivers do similar work, but they raise a question mountains don’t: where inside the river does the border sit?
Modern practice answers with the thalweg, a German term for the line of the deepest or most navigable channel. Before the nineteenth century, river borders typically ran down the geometric middle, but that rule created problems whenever the channel shifted. Riparian states moved to the thalweg as the more practical alternative, and it is now the default in most river-boundary treaties.1Cambridge Core. Notes on Rivers as Boundaries Deserts, lakes, and coastlines fill the same role elsewhere, each with its own complications when the landscape changes.
Straight Lines and Treaty Borders
Plenty of borders ignore the land entirely. They appear as straight lines drawn along parallels or meridians, with no regard for rivers or ridges. The border between the United States and Canada west of the Great Lakes follows the 49th parallel for roughly 2,000 miles. Much of Africa’s political map is made up of similar ruler-straight lines, a legacy of European colonial powers partitioning the continent at conferences in the late 1800s.
Those geometric borders carry a cost. Colonial administrators drew them to divide spheres of influence, often cutting through ethnic groups, trade routes, and ecosystems. Local realities rarely factored in. Many territorial disputes that persist today trace directly to those decisions, and the fact that the lines have mostly survived independence has less to do with their wisdom than with a legal principle, discussed further below, that discourages redrawing them.
Most borders on the map today, whether they follow geography or not, were fixed or confirmed by treaty. A treaty codifies an agreed line and, once ratified, binds both parties under international law. Some emerge from post-conflict settlements. Others formalize borders that existed informally for decades or centuries. Historical claims carry real weight at the negotiating table: length of control, ethnic roots, colonial-era administrative assignments, prior agreements, and maps all get invoked. The diplomats’ job is to reconcile competing claims into a line both sides can accept.
Treaties sometimes produce odd geographic results. An enclave is a piece of one country’s territory entirely surrounded by another; an exclave is the same territory seen from the country it belongs to. Until a land-swap agreement in 2015, the India-Bangladesh border contained over 100 enclaves, some nested inside each other.
When a River Moves
Rivers meander, flood, and cut new channels. International law handles this through two concepts.
Accretion is the slow, gradual process by which a river deposits soil on one bank and erodes the other. When it happens, the border moves with the channel. An 1884 boundary convention between the United States and Mexico set out this rule explicitly, keeping the border in the river’s normal channel despite changes, so long as those changes came from slow erosion and natural deposit rather than a sudden jump.1Cambridge Core. Notes on Rivers as Boundaries
Avulsion is the opposite. A flood or similar sudden event abandons the old bed and carves a new one. When that happens, the border stays where the old channel was, and the country that lost land to the shift keeps its territory on paper. The distinction matters in practice, because a single flood can move a river miles from its previous course.
Borders at Sea
More than two-thirds of the Earth’s surface is water, and the rules for dividing it are among the most precisely defined in international law. The United Nations Convention on the Law of the Sea, which 170 nations have joined, sets up a series of concentric zones extending outward from a coastline, each carrying different rights.
Territorial Sea and Contiguous Zone
Every coastal nation can claim a territorial sea reaching up to 12 nautical miles from its baseline, generally the low-water line along the coast.2United Nations. UNCLOS Part II – Territorial Sea and Contiguous Zone Inside this zone the state has full sovereignty, as it does over its land. Foreign vessels get a right of innocent passage, but the coastal state controls nearly everything else.
Beyond that lies the contiguous zone, out to 24 nautical miles from the baseline. Sovereignty is not full here, but the state can enforce customs, immigration, tax, and sanitary regulations to prevent violations within its territory or territorial sea.2United Nations. UNCLOS Part II – Territorial Sea and Contiguous Zone
Exclusive Economic Zone and Continental Shelf
The exclusive economic zone reaches up to 200 nautical miles from the baseline. Inside the EEZ, the coastal state holds sovereign rights over natural resources in the water, on the seabed, and beneath it, including fish, oil, gas, and energy from wind and currents.3United Nations. UNCLOS Part V – Exclusive Economic Zone It also has jurisdiction over artificial islands, marine research, and environmental protection. Other nations can navigate and fly through, but they cannot extract resources without permission.
The continental shelf covers the seabed and subsoil extending from the coast along the natural prolongation of the land mass. It runs 200 nautical miles by default, and a nation whose geological shelf goes further can claim rights beyond that distance after submitting scientific evidence to a UN commission.4United Nations. UNCLOS Part VI – Continental Shelf Some of the most expensive boundary disputes play out at this layer, because the stakes involve offshore oil and gas reserves worth billions.
Borders in the Air
Borders extend upward as well as outward. The 1944 Chicago Convention established that every state has complete and exclusive sovereignty over the airspace above its territory. A nation’s aerial boundary mirrors its ground-level and maritime boundaries, and no aircraft can enter without permission. Airlines negotiate overflight rights bilaterally, and unauthorized entry can trigger a military response.
Where sovereign airspace ends and outer space begins is unsettled. No binding agreement draws the line, but the most widely discussed proposal places it at the Kármán line, roughly 100 kilometers above sea level, the altitude at which an aircraft can no longer generate aerodynamic lift and would need orbital velocity to stay aloft.5United Nations Office for Outer Space Affairs. Definition and Delimitation of Outer Space Several states endorse this boundary, but formal consensus is elusive. In practice, satellites orbiting above 100 kilometers are generally treated as being in international space.
The Legal Principles That Keep Borders Where They Are
Two principles have shaped more borders than any single treaty negotiation.
Uti Possidetis Juris
The phrase means, roughly, “as you possess under law.” Newly independent states inherit the administrative boundaries drawn by their former colonial rulers, even where those boundaries are arbitrary or poorly suited to the new political reality. The principle originated in nineteenth-century Latin America, when former Spanish colonies needed a rule to prevent territorial chaos as they gained independence.6Legal Information Institute. Uti Possidetis Juris
The International Court of Justice later recognized it as a general principle applicable wherever independence occurs. In the 1986 Burkina Faso/Mali frontier dispute, the court explained that its purpose is to prevent “fratricidal struggles provoked by the challenging of frontiers following the withdrawal of the administering power.”6Legal Information Institute. Uti Possidetis Juris African nations applied it widely during decolonization, and it was invoked again when the Soviet Union and Yugoslavia dissolved in the 1990s.
Self-Determination
The UN Charter lists among the organization’s core purposes the development of relations based on “equal rights and self-determination of peoples.”7United Nations. Purposes and Principles of the UN – Chapter I of UN Charter In practice, the principle has supported decolonization movements and, more rarely, the creation of new states.
Self-determination and uti possidetis juris sit in obvious tension. One says peoples should choose their own political fate; the other says existing boundaries should be preserved. International law has generally resolved the tension in favor of stability. Self-determination most often operates within existing borders rather than by creating new ones. When new states do emerge, as South Sudan did in 2011, the process typically involves negotiated agreements and international recognition rather than unilateral border changes.
How Border Disputes Get Resolved
When two countries disagree about where a border belongs, international law offers several paths short of war.
Direct negotiation is the starting point. Countries exchange maps, present historical evidence, and try to reach a deal between themselves. When talks stall, they can bring in a mediator, a neutral third party who facilitates discussion and may propose solutions but cannot impose them. Mediation keeps both sides in control of the outcome, which makes it more politically workable than the alternatives.
Arbitration goes a step further. Both countries agree in advance to submit their dispute to a neutral panel and to be bound by whatever decision comes back. The Permanent Court of Arbitration, established in 1899, administers many of these proceedings, and its awards are final and binding.8United Nations. Permanent Court of Arbitration – Settlement of Disputes The Eritrea-Ethiopia boundary dispute was resolved through a binding arbitration commission.
The highest-profile option is adjudication by the International Court of Justice, the principal judicial organ of the United Nations. The ICJ can hear a border dispute when both states accept its jurisdiction, either through a special agreement, a treaty clause, or standing declarations recognizing the court’s authority.9International Court of Justice. How the Court Works Its judgments are final and without appeal. The court’s docket includes dozens of territorial and maritime cases, from the Nicaragua-Colombia maritime delimitation to the Gabon-Equatorial Guinea land and maritime boundary case.10International Court of Justice. Contentious Cases The catch is consent. A country cannot be dragged before the court against its will.
From an Agreed Line to a Marked Border
Agreeing on a border is one thing. Making it visible on the ground is another. Demarcation is the physical process of marking an agreed boundary with pillars, fences, cairns, cleared vegetation, or other visible indicators. Without it, even a legally settled border can generate confusion and low-level conflict.
Joint boundary commissions handle this work. The U.S.-Canada International Boundary Commission maintains the border vista, a 20-foot-wide clear-cut strip running through forests along the boundary line. Commissions survey and re-survey the line, replace damaged markers, and resolve minor discrepancies as landscapes change. Modern commissions increasingly rely on GPS coordinates and satellite data to supplement the traditional physical monuments with digital reference points. Maintaining a border is an ongoing obligation, not a one-time event, and neighboring countries divide the cost by treaty on the principle that each side pays for what benefits it.