What are the oldest international borders in the world?

Portugal's frontier with Spain is often named among the world's oldest surviving international borders, with much of its territorial framework dating to the Treaty of Alcañices in 1297. But a single “oldest border” is harder to identify than a list of treaty dates suggests.

The answer changes depending on whether we count a boundary's general location or its precise surveyed course, include microstates, or allow lines that no longer separate independent countries. Portugal and Spain, Andorra, San Marino and the England–Scotland boundary illustrate those different questions.

Why It Is Difficult to Define the "Oldest Border"

Modern international borders are normally understood as exact legal lines dividing the sovereign territories of states. This concept developed gradually. Medieval rulers often exercised authority through overlapping feudal rights, personal allegiances, ecclesiastical jurisdictions and zones of influence. A frontier might consist of forests, rivers, mountain valleys or fortified settlements rather than a precisely surveyed line. Even when medieval treaties described territorial boundaries, the geographical precision expected from a modern border was often absent. Local communities might possess grazing rights on both sides, nobles could hold estates under different rulers, and villages might dispute particular fields for centuries. The development of accurate surveying and modern cartography eventually transformed these frontier zones into more precisely demarcated boundaries.

There is another difficulty: the countries themselves changed. France, Portugal and England existed as recognizable medieval kingdoms, but their political institutions were fundamentally different from those of their modern successor states. Other present-day countries emerged much later from older kingdoms, empires or federations. A boundary could therefore be older than the modern constitutional form of either state that it now separates. The most defensible examples of ancient borders are consequently those where a medieval or early modern territorial settlement can be connected relatively continuously to a present international boundary. By that criterion, western Europe contains several exceptional cases because political consolidation occurred relatively early and some states avoided the repeated territorial reconstruction experienced in central and eastern Europe.

Portugal and Spain: The Most Famous Candidate

The border between Portugal and Spain is frequently described as one of the oldest international borders in the world and, with appropriate qualifications, it is probably the strongest example of a very old European state boundary surviving into the present. Portugal emerged as a separate kingdom during the twelfth century in the context of the Christian expansion against Muslim-controlled territories in the Iberian Peninsula. Portuguese independence from the Kingdom of León was recognized during the twelfth century, but the territorial limits of the new kingdom continued to evolve as Portuguese and other Iberian rulers expanded southward. A decisive moment came with the Treaty of Alcañices in 1297. King Denis of Portugal and King Ferdinand IV of Castile reached an agreement settling important territorial disputes and establishing much of the frontier between Portugal and the neighboring Castilian kingdom. Many sections of the modern Portuguese-Spanish border ultimately derive from this settlement.

The border is commonly known in Portuguese and Spanish as La Raya or A Raia, meaning simply "the line." It extends for more than 1,200 kilometers from the Atlantic coast in the north to the mouth of the Guadiana in the south.

However, saying that the entire modern Portugal–Spain border has remained completely unchanged since 1297 would be inaccurate. There have been later territorial modifications and disputes. The most famous concerns Olivenza, or Olivença, which Spain occupied in 1801. Portugal has historically maintained a claim concerning the territory, although Spain administers it. Other sections of the frontier were subsequently clarified through modern boundary treaties.

Andorra: A Medieval Political Arrangement That Survived

Another remarkable example can be found in the Pyrenees. Andorra is a small state located between France and Spain whose origins are deeply connected to medieval feudal arrangements. The crucial agreements were the paréages of 1278 and 1288. These agreements resolved a dispute over Andorra between the Bishop of Urgell and the Count of Foix by establishing a form of shared lordship. The unusual political arrangement eventually evolved into the Andorran co-principality.

One of the medieval lines of succession ultimately connected the rights of the Count of Foix to the French monarchy and later to the French head of state. Consequently, modern Andorra still possesses two co-princes: the Bishop of Urgell and the President of France.

The country's survival means that boundaries surrounding Andorra preserve a territorial unit whose political distinctiveness dates back to the Middle Ages. Its borders with France and Spain therefore belong among Europe's oldest surviving international boundary systems.

San Marino and the Survival of a Microstate

San Marino provides an even more dramatic example of institutional longevity, although the history of its exact borders requires careful distinction from the legendary age of the state itself. According to tradition, San Marino was founded in AD 301 by Marinus, a Christian stonemason. This date forms an important part of the republic's historical identity, but it should not be interpreted as evidence that the present international boundary has existed unchanged since the fourth century.

The territorial development of San Marino occurred gradually during the Middle Ages. Its boundaries expanded at various points, particularly during the fifteenth century. In 1463, after participating in a conflict against the Malatesta family, San Marino acquired several territories that helped give the republic approximately its present territorial extent.

Its modern international border is therefore entirely surrounded by Italy, but the territorial outline of the republic is substantially older than Italy itself. The Kingdom of Italy was proclaimed only in 1861, centuries after San Marino had acquired approximately its modern boundaries.

France and Spain: The Pyrenees as a Long-Lived Frontier

The Pyrenees have separated political territories on the Iberian Peninsula from those north of the mountains for many centuries, but the modern France–Spain border cannot simply be projected back unchanged into the Middle Ages. A major turning point came with the Treaty of the Pyrenees in 1659, which ended a long conflict between the French and Spanish monarchies. Spain transferred several territories to France, including most of Roussillon and parts of the Cerdanya region. The settlement helped establish the broad framework of the modern frontier. Later agreements were still necessary to define sections of the border more precisely. During the nineteenth century, a series of treaties known collectively as the Treaties of Bayonne clarified and demarcated much of the frontier. Consequently, the modern border combines an old geopolitical division with later technical delimitation.

The frontier also contains unusual survivals of older territorial arrangements. One of the most remarkable is Llívia, a Spanish municipality surrounded by French territory. When Spain transferred villages in Cerdanya to France after the Treaty of the Pyrenees, Llívia was treated as a town rather than a village and remained Spanish. It consequently became an enclave of Spain inside France. Another curiosity is Pheasant Island in the Bidasoa River, whose sovereignty alternates between France and Spain during the year under a condominium arrangement. Such anomalies illustrate how centuries of treaties can produce borders that are historically old but legally and geographically complicated.

England and Scotland: An Ancient Border That Became an Internal Boundary

The boundary between England and Scotland presents a different problem because it is extremely old but is no longer an international border between sovereign states. England and Scotland developed as separate kingdoms during the Middle Ages, and their frontier was contested through centuries of warfare. The Treaty of York of 1237 established an important territorial settlement between the English and Scottish crowns and helped define a boundary broadly resembling the later border. Further disputes continued, particularly around the western frontier, but by the early modern period the basic territorial division between England and Scotland was well established. When James VI of Scotland inherited the English throne in 1603, the two kingdoms acquired the same monarch but remained legally separate states.

The Acts of Union of 1707 then united England and Scotland into the Kingdom of Great Britain. Their ancient international frontier became an internal administrative and political boundary.

The England–Scotland line is therefore potentially older than several boundaries commonly described as among the world's oldest international borders, but it no longer meets a strict definition of an international boundary. It nevertheless demonstrates how old borders can survive even after the states they originally separated unite into a larger country.

Scandinavia and the Survival of Early Modern Boundaries

Northern Europe also contains several old international boundaries, although repeated wars and political unions make their chronology complicated.

The border between Norway and Sweden was substantially defined in the eighteenth century. The Treaty of Strömstad of 1751 established much of the boundary and was accompanied by arrangements concerning Sámi use of territories across the frontier. When Norway separated from its union with Sweden in 1905, the existing territorial boundary became an international border between fully separate states. The Sweden–Finland border has a different history. Finland formed part of the Swedish realm for centuries before Sweden ceded it to the Russian Empire in 1809. The boundary between Sweden and Russian Finland was established primarily along the Torne and Muonio rivers. When Finland became independent in 1917, this former imperial boundary became the modern Sweden–Finland border.

Why Eastern and Central Europe Have Fewer Extremely Old Borders

The relative abundance of old boundaries in parts of western Europe contrasts sharply with the history of central and eastern Europe. This does not mean that political territories there lacked ancient frontiers. Rather, the region experienced repeated transformations that frequently erased or relocated international boundaries. The Polish-Lithuanian Commonwealth was partitioned by Russia, Prussia and Austria at the end of the eighteenth century. The Holy Roman Empire contained hundreds of political units before its dissolution in 1806. The unifications of Germany and Italy during the nineteenth century transformed numerous former international borders into internal boundaries. The First World War destroyed the German, Austro-Hungarian, Russian and Ottoman empires, while the Second World War produced further enormous territorial changes.

Poland provides one of the clearest examples. Although the Polish state has medieval origins, its present western border along the Oder and Neisse rivers dates from the territorial settlement associated with the end of the Second World War, while its eastern border reflects the postwar Soviet-Polish settlement and subsequent emergence of independent Lithuania, Belarus and Ukraine.

Colonial Borders and the Problem of State Succession

Outside Europe, determining the oldest borders becomes even more complicated because many present international boundaries originated during European colonial rule. Large parts of the Americas, Africa, Asia and Oceania were divided into colonial territories whose administrative boundaries later became borders between independent states. A modern country might therefore be only several decades or centuries old as a sovereign state while its boundary had already existed for much longer as a colonial division.

The United States–Canada border provides a good example. Much of the famous western boundary follows the 49th parallel as a result of British-American agreements reached in 1818 and 1846. Canada itself was not created as a federation until 1867. The international line separating American territory from British North America therefore predates modern Canada.

In Latin America, many international borders originated from the administrative divisions of the Spanish and Portuguese empires, but translating those colonial jurisdictions into precise independent-state boundaries produced numerous nineteenth- and twentieth-century disputes. Similarly, most African countries retained colonial borders when they became independent during the twentieth century.

What Actually Counts as the Oldest Border?

For a long present-day international frontier with a recognizably medieval territorial framework, Portugal and Spain offer a strong candidate. The Treaty of Alcañices of 1297 was a major settlement, although later agreements and surveys refined the boundary and some questions remained contested.

Microstates complicate the comparison. Andorra's arrangement dates to the thirteenth century, while San Marino reached roughly its present extent in the fifteenth. England and Scotland preserve another medieval division, but their border has been internal to one sovereign state since 1707.

A useful comparison must therefore state what is being dated: a treaty, a stretch of territory, a demarcated line or an uninterrupted international boundary. Without that distinction, declaring a world record gives a misleading impression of precision.