Why country borders look the way they do — colonialism, rivers, and arbitrary lines

Look at any world map for thirty seconds and you'll notice two things. First, a lot of borders follow rivers and mountain ranges. Second, an absurd number of them are perfectly straight lines that have no business being on a globe. The straight lines are the giveaway: somebody, somewhere, drew them on a piece of paper without consulting the people on the ground. The crooked ones are usually the older borders, where geography did the talking. Most national borders fall into one of four categories, and once you can tell which is which, the political map of the world starts to make a kind of grim sense.

Category 1: rivers and watersheds

The oldest natural border is a river. Rivers are easy to find, hard to argue about, and tend to mark a real cultural break — the people on one bank usually farm differently, fish differently, and trade differently from the people on the other. The Rhine has separated Latin Europe from Germanic Europe for two millennia. The Rio Grande/Río Bravo separates the United States from Mexico for roughly 2,000 km. The Danube has been a border between empires (Roman, Ottoman, Austro-Hungarian) more or less continuously for 2,000 years.

The problem with rivers is that they move. Rivers shift course over centuries — sometimes over decades. The Mississippi has changed channel multiple times in recorded history; the Hwang Ho (Yellow River) in China has shifted hundreds of kilometres. When a border river moves, you get a border dispute. The United States and Mexico have had multiple Rio Grande boundary disputes over which side of an oxbow lake belongs to whom; the most famous, the Chamizal dispute, took 100 years to resolve. The general principle in international law is that the border follows the thalweg — the deepest navigable channel — and that this channel is fixed at the time of the treaty even if the river later moves. In practice, this is interpreted differently in different treaties, and lawyers have made careers out of the ambiguity.

Category 2: mountain ranges and watersheds

Mountains are even more durable borders than rivers. The Pyrenees have been the Spain-France border for so long that it's difficult to imagine an alternative. The Alps separate Italy from Switzerland, Austria, and France. The Andes separate Chile from Argentina along the longest mountain border in the world (over 5,000 km). The Himalayas separate the Indian subcontinent from the Tibetan plateau, and the practical impassability of the high passes is one of the reasons that India and China developed as such separate civilizations despite being neighbours.

Mountain borders are typically defined by the watershed — the highest line, on the assumption that water flowing west belongs to one country and water flowing east belongs to the other. This is mostly clean, but the Andes border between Chile and Argentina has had persistent disputes in places where the highest peak and the watershed don't quite line up — for example, around the Patagonian icefields, where the geography is genuinely ambiguous and 50 km of border was only formally settled in 1998.

Category 3: lines of latitude and longitude

When you see a border that runs perfectly east-west or north-south, you are almost always looking at the work of European colonial powers in the 19th and early 20th centuries. The 49th parallel marks a 2,000 km stretch of the United States-Canada border, drawn by the 1818 Convention between Britain and the US, with both governments having only the vaguest idea what was actually on the ground. The 38th parallel was drawn across the Korean peninsula in 1945 in about 30 minutes by two American officers who had a National Geographic map and were trying to divide the country with the Soviets. That story is well-documented .

The most famous example, though, is Africa. At the Berlin Conference of 1884-85 , European powers carved up the continent without sending anyone to consult the people who lived there. Most of the African colonial borders that became national borders in the 1960s — and which are still in place today — were drawn with rulers and protractors on incomplete maps. This is why so many African borders are dead-straight lines: between Algeria and Libya, between Mali and Mauritania, between Egypt and Sudan. The lines are administrative artefacts, not reflections of where one ethnic group ended and another began. The post-independence project of "African nation-building" has, in many places, been the project of trying to make a colonial border into a national identity.

The same pattern appears, more recently, in the Middle East. The Sykes-Picot Agreement of 1916 between Britain and France carved up the post-Ottoman Arab world along straight lines drawn through what the negotiators considered "neutral desert." Many of the resulting borders — the Iraq-Saudi Arabia border, the Iraq-Jordan border, the Syria-Iraq border — are still essentially the Sykes-Picot lines. The persistent instability of those borders, and of the states they enclose, is not unrelated to how they were drawn.

Category 4: wars, treaties, and accidents

The fourth category is everything else: borders that follow neither geography nor a clean colonial line, because they were the residue of a specific war or treaty. These are the squiggly, illogical borders that don't make sense until you read the history.

The bizarre border between the United States and Canada at the Northwest Angle — a small piece of Minnesota that is north of the 49th parallel and accessible from the rest of the US only by boat or by driving through Canada — is the result of a 1783 treaty drawn from a map that turned out to be wrong about the source of the Mississippi. The negotiators specified that the border would run westward from the source of the river, but the river didn't actually go where the map said it did. The fix was to add a small dogleg, and 250 years later the dogleg is still there.

The Caprivi Strip in Namibia — a 450 km long strip of land sticking out to the east — exists because the German Empire wanted access to the Zambezi River so it could (in theory) reach the east coast of Africa. The strip was negotiated with Britain in 1890, the practical access never materialised, but the strip remained when Namibia became independent in 1990.

The enclaves of Baarle , on the Belgium-Netherlands border, are 30-something tiny patches of Belgium inside the Netherlands and a few patches of Netherlands inside the patches of Belgium. They're the residue of medieval feudal land deals between local lords that were never tidied up and survived through every subsequent reorganisation of European borders. They're extraordinarily strange to walk around — café tables can have a national border running through them.

Bir Tawil , a 2,000 sq km piece of desert between Egypt and Sudan, is one of the only pieces of dry land on Earth that neither country claims, because both countries claim a different (and more valuable) territory nearby — the Hala'ib Triangle — and accepting Bir Tawil would mean conceding the triangle. So Bir Tawil is, on paper, terra nullius — unclaimed territory. There's no border post.

What this means for the quiz

If you've ever played the Find on Map quiz on hard mode and wondered why the borders of Sudan, Chad, and Niger are so easy to confuse — that's why. They're Berlin Conference borders, drawn straight, and they don't correspond to natural features. The borders of, say, Norway and Sweden are much easier to remember because they follow the spine of the Scandinavian Mountains; the border between Brazil and Bolivia is partially the Amazon's tributaries; the border between Germany and Poland is the Oder and the Neisse rivers. Geography-driven borders are easier to learn because they have a story. Treaty-driven borders are harder because the only story is "someone drew this in London in 1894."

There's also a practical lesson for the Guess the Shape quiz: the most distinctive country shapes in the world are almost all geography-driven (Italy's boot, Norway's spine, Chile's strip, Vietnam's S). The hardest-to-identify shapes are the rectangular African and Middle Eastern colonial polygons ( Egypt, Libya, Algeria, Mali, Niger, Chad, Sudan, Saudi Arabia — almost all interchangeable at first glance). The ones with rivers and coastlines stick in your memory; the ones with ruler-edges blur together.

Borders that are still moving

The world map is not finished. South Sudan voted to leave Sudan in 2011 — the most recent country to be created. The dissolution of Yugoslavia produced seven new states across the 1990s and 2000s. Czechoslovakia split into the Czech Republic and Slovakia in 1993, peacefully and almost as a footnote. There are active border disputes today between India and China (the Aksai Chin and Arunachal Pradesh), between Russia and Ukraine (multiple), between Israel and its neighbours (multiple), between Venezuela and Guyana, between Ethiopia and Eritrea, and a long list of smaller ones.

If you're memorising the world map for a quiz, the version that's correct in 2026 will probably need a small update by 2036. The major shape will be the same — the rivers, mountains, and 19th-century colonial lines have inertia — but expect a few more straight-line borders to wobble or split, and at least one new country before 2050 (Greenland, Catalonia, Scotland, Bougainville, and Western Sahara are all plausible candidates, though none is certain).