The Enduring Influence of Treaty-Making on European Boundaries

Treaties have functioned as the primary legal instruments for redefining European borders for centuries. From the Peace of Westphalia in the 17th century to the agreements that followed the dissolution of the Soviet Union, these documents have redrawn maps, transferred populations, and established the sovereignty of states. The borderlines visible on today’s map of Europe are not arbitrary; they are the accumulated result of countless diplomatic negotiations, military outcomes, and political compromises codified in treaty language. Understanding the trajectory of these changes is essential for grasping contemporary national identities, regional tensions, and the legal frameworks that govern European cooperation.

The process of border alteration through treaties is rarely clean or universally accepted. Each agreement reflects the power dynamics of its era, the ambitions of major states, and the often-overlooked aspirations of smaller nations. Some treaties created entirely new countries, others erased them from the map, and many shifted populations across newly drawn lines. The consequences of these decisions continue to resonate in modern disputes, cultural landscapes, and economic relationships across the continent.

The Foundations of Modern State Borders: Early Treaties

The concept of clearly defined, sovereign state borders emerged gradually in Europe. Before the early modern period, borders were often vague, overlapping zones of influence rather than precise lines. Treaties from the 17th and 18th centuries began to change this by establishing more formal territorial arrangements.

The Peace of Westphalia (1648)

The Peace of Westphalia, which ended the Thirty Years’ War, is widely regarded as the foundational document of the modern state system. The treaties of Osnabrück and Münster established several principles that would shape European borders for centuries. Most importantly, they affirmed the concept of territorial sovereignty, meaning that each ruler had control over the affairs within their own borders without external interference. This principle directly led to the recognition of the independence of the Swiss Confederacy and the Dutch Republic, formalizing borders that had been contested for decades. The peace also reorganized the Holy Roman Empire, granting greater autonomy to its constituent states and reducing the power of the Emperor. The Westphalian model became the template for subsequent European peace settlements.

The Treaty of Utrecht (1713)

The Treaty of Utrecht, which ended the War of the Spanish Succession, had significant territorial consequences. Philip V was recognized as King of Spain on the condition that the French and Spanish crowns would never be united, preventing a superpower that would have dominated Western Europe. Territorially, the treaty transferred Gibraltar and Minorca to Great Britain, established Austrian control over the Spanish Netherlands (roughly modern-day Belgium), and gave the Duchy of Savoy the Kingdom of Sicily, which later became the Kingdom of Sardinia. These adjustments created long-lasting border arrangements in the Mediterranean and the Low Countries that would persist in some form for centuries. The treaty also extended British colonial holdings in North America, but its primary European impact was the establishment of a balance of power that limited French expansion.

The Congress of Vienna (1815)

After the Napoleonic Wars, the Congress of Vienna undertook one of the most comprehensive redrawings of European borders in history. Under the guidance of Prince Metternich of Austria and other leading statesmen, the Congress aimed to restore stability and contain French aggression. The settlement created a new map that included the following major changes:

  • Confederation of the Rhine was dissolved and replaced by the German Confederation, a loose association of 39 German states that replaced the defunct Holy Roman Empire.
  • France was reduced to its 1790 borders and required to pay an indemnity, though the allies avoided overly punitive measures to ensure long-term stability.
  • The Kingdom of the Netherlands was created by merging the Dutch Republic with the Austrian Netherlands (Belgium), forming a buffer state north of France.
  • Norway was transferred from Denmark to Sweden as compensation for Swedish losses in Finland.
  • Russia gained most of the Duchy of Warsaw, which became the Kingdom of Poland under Russian control.
  • Prussia received significant territories in western Germany and parts of Saxony, positioning it as a major German power.

The Congress of Vienna established a territorial order that largely held until the mid-19th century and provided the framework for the great-power diplomacy that followed.

National Unification and Imperial Decline: The 19th Century Treaties

The 19th century saw the rise of nationalism as a dominant political force. Treaties from this period often reflected the tension between existing imperial boundaries and the growing demand for nation-states based on linguistic or ethnic identity.

The Treaty of Paris (1856)

The Treaty of Paris, which ended the Crimean War, had two important border-related outcomes. First, it neutralized the Black Sea, prohibiting Russia from maintaining a naval presence there and limiting its influence in the Balkans. This created a security framework that temporarily reduced Russian pressure on the Ottoman Empire. Second, the treaty required Russia to return the city of Bessarabia (modern-day Moldova) to the Ottoman Empire. Although these provisions were later reversed, the treaty demonstrated how great-power agreements could reshape the geopolitical landscape of Eastern Europe and the strategic balance of power in the Mediterranean region.

The Treaty of Berlin (1878)

The Treaty of Berlin was a comprehensive settlement that addressed the territorial consequences of the Russo-Turkish War of 1877-1878. It recognized the independence of Serbia, Montenegro, and Romania, establishing them as sovereign states with defined borders. The treaty also created Bulgaria as an autonomous principality under Ottoman suzerainty, though its size was significantly reduced from the earlier San Stefano treaty to prevent Russian dominance in the Balkans. Bosnia and Herzegovina was placed under Austro-Hungarian administration while technically remaining part of the Ottoman Empire, a decision that would have major consequences leading to World War I. Additionally, Cyprus was placed under British administration. The Treaty of Berlin dramatically altered the map of Southeast Europe and set the stage for the Balkan Wars and World War I.

Unification Treaties: Germany and Italy

The creation of Germany and Italy as unified states involved a series of treaties that redrew the borders of Central and Southern Europe. The Treaty of Frankfurt (1871), which ended the Franco-Prussian War, resulted in the annexation of the French territories of Alsace and Lorraine by the newly proclaimed German Empire. This border change created a lasting source of tension between France and Germany that would contribute to World War I. Meanwhile, Italian unification was achieved through a series of diplomatic agreements and plebiscites that gradually brought the various Italian states under the rule of the Kingdom of Sardinia. The Treaty of Vienna (1866) ceded the Veneto region to Italy, and the later Treaty of Rome (1889) formalized the borders of the Kingdom of Italy after the incorporation of the Papal States.

World War I and the Treaty of Versailles System

The end of World War I brought about the most dramatic border changes in European history up to that point. The collapse of the German, Austro-Hungarian, Russian, and Ottoman Empires created a power vacuum that was filled by a series of treaties collectively known as the Versailles System.

The Treaty of Versailles (1919)

The Treaty of Versailles with Germany imposed harsh territorial losses. The following areas were separated from the German Empire:

  • Alsace-Lorraine was returned to France, restoring the pre-1871 border.
  • Eupen and Malmedy were transferred to Belgium after a controversial local plebiscite.
  • Northern Schleswig was returned to Denmark after a referendum.
  • The Saar Basin was placed under League of Nations administration for 15 years and its coal mines were given to France.
  • Danzig (modern-day Gdańsk) was established as a Free City under League of Nations protection, giving Poland access to the sea without annexing the German-speaking city.
  • Upper Silesia was partitioned between Germany and Poland after a plebiscite and subsequent League of Nations decision.
  • The Memel Territory was separated from Germany and later annexed by Lithuania.

Germany also lost all of its overseas colonies. The eastern border of Germany was redrawn to create the Polish Corridor, which gave Poland access to the Baltic Sea but separated East Prussia from the rest of Germany. This territorial settlement created significant resentment in Germany and became a focus of Nazi revanchist demands.

The Treaty of Saint-Germain (1919)

The Treaty of Saint-Germain with Austria dissolved the Austro-Hungarian Empire and established the borders of the Republic of Austria. Austria was reduced to a small, landlocked state of approximately 84,000 square kilometers, with a population of about 6.5 million. The treaty explicitly forbade Anschluss (union with Germany), a provision that would later be violated by Hitler. Key territorial losses included:

  • South Tyrol, Trentino, Istria, and the city of Trieste were ceded to Italy, fulfilling Italian irredentist claims.
  • Bohemia, Moravia, and Silesia became part of the newly created Czechoslovakia.
  • Galicia was incorporated into the reestablished Poland.
  • Bukovina was transferred to Romania.
  • Slovenia, Croatia, and Bosnia and Herzegovina joined with Serbia and Montenegro to form the Kingdom of Serbs, Croats, and Slovenes (later Yugoslavia).

The treaty left Austria as a rump state that struggled economically and politically, contributing to the instability that led to the rise of fascism.

The Treaty of Trianon (1920)

The Treaty of Trianon with Hungary was one of the most punitive of the Versailles System. Hungary lost approximately two-thirds of its territory and about three-fifths of its population. The major territorial cessions were as follows:

  • Transylvania and parts of the Banat were awarded to Romania.
  • Slovakia and Carpathian Ruthenia became part of Czechoslovakia.
  • Burgenland was transferred to Austria.
  • Slavonia, Croatia, and part of the Banat went to the Kingdom of Serbs, Croats, and Slovenes.

The Treaty of Trianon left approximately 3 million ethnic Hungarians living outside Hungary’s new borders, creating an enduring source of revisionist sentiment in Hungarian politics. The treaty’s harsh terms continue to influence Hungarian-Romanian and Hungarian-Slovak relations today.

The Treaty of Neuilly (1919) and the Treaty of Sèvres (1920) / Lausanne (1923)

The Treaty of Neuilly with Bulgaria resulted in the loss of Western Thrace to Greece and parts of Dobruja to Romania, cutting Bulgaria off from the Aegean Sea. The Treaty of Sèvres with the Ottoman Empire was never fully implemented due to the Turkish War of Independence. It was replaced by the Treaty of Lausanne (1923), which established the borders of modern Turkey. This treaty was notable for its provision for a population exchange between Greece and Turkey, affecting approximately 1.5 million Greeks and 500,000 Turks. The Lausanne settlement created the modern Turkish state’s boundaries and largely ended territorial disputes in the Eastern Mediterranean, though the status of Cyprus and the Aegean islands remained contentious.

World War II and the Postwar Settlement

World War II caused even more extensive border changes than World War I. The postwar treaties, particularly the Potsdam Agreement and the Treaties of Paris (1947), redrew the map of Europe with profound consequences that shaped the Cold War order.

The Potsdam Agreement (1945)

The Potsdam Agreement, signed by the United States, the United Kingdom, and the Soviet Union, established the framework for postwar Europe. Its most significant border-related provisions concerned Germany and Poland.

  • Germany’s eastern border was shifted westward to the Oder-Neisse line, placing large territories that had been part of Germany under Polish and Soviet administration.
  • East Prussia was partitioned between Poland and the Soviet Union, with the northern half becoming the Kaliningrad Oblast (now a Russian exclave).
  • Poland’s borders were moved westward: it lost its eastern territories to the Soviet Union but gained German territories in the west and north.
  • German populations in Central and Eastern Europe were expelled in an organized transfer that affected approximately 12 million people, one of the largest population movements in history.

The Potsdam Agreement also confirmed the division of Germany into four occupation zones, which later evolved into West Germany and East Germany. The Oder-Neisse line became a permanent border and was formally recognized in the 1990 Treaty on the Final Settlement with Respect to Germany (the Two Plus Four Agreement).

The Treaties of Paris (1947)

The 1947 treaties with Italy, Romania, Hungary, Bulgaria, and Finland imposed territorial adjustments that further consolidated the postwar order. Key changes included:

  • Italy lost all its colonial possessions and ceded Istria, Zara, and most of the Julian March to Yugoslavia. The Free Territory of Trieste was created (later divided between Italy and Yugoslavia). Italy also lost the Dodecanese Islands to Greece.
  • Romania ceded northern Bukovina and Bessarabia to the Soviet Union, but regained northern Transylvania from Hungary.
  • Hungary returned to its Trianon borders, confirming the loss of territories gained during the war.
  • Bulgaria retained southern Dobruja, which it had gained from Romania during the war, and kept its borders largely intact.
  • Finland ceded the Petsamo region to the Soviet Union and leased the Porkkala Peninsula as a naval base for 50 years (returned in 1956).

These treaties formalized the Soviet sphere of influence in Eastern Europe and created the border framework that would persist until the collapse of communism.

The Cold War and Its Conclusion

During the Cold War, European borders were largely frozen by the division between NATO and the Warsaw Pact. The Helsinki Final Act (1975) of the Conference on Security and Co-operation in Europe affirmed the inviolability of frontiers, effectively recognizing the postwar borders of Europe. This agreement was a compromise: the West accepted the territorial status quo in exchange for commitments to human rights and cooperation. The Helsinki process did not prevent future changes, but it established a framework for peaceful border adjustments rather than violent revision.

The Reunification of Germany (1990)

The end of the Cold War brought about the most significant peaceful border change in modern European history: the reunification of Germany. The Treaty on the Final Settlement with Respect to Germany, also known as the Two Plus Four Treaty, was signed by East Germany, West Germany, and the four Allied powers (United States, United Kingdom, France, and the Soviet Union). The treaty confirmed the finality of the Oder-Neisse line as Germany’s eastern border, renounced any future territorial claims, and allowed a unified Germany to regain full sovereignty. This was a landmark agreement that resolved the German question that had been a source of conflict since the 19th century.

The Dissolution of the Soviet Union and Yugoslavia

The collapse of the Soviet Union and Yugoslavia led to the creation of 15 new European states. These changes were governed by a combination of unilateral declarations of independence, international recognition, and bilateral treaties. The Belavezha Accords (1991) formally dissolved the Soviet Union, while the Alma-Ata Protocol established the Commonwealth of Independent States. The borders of the newly independent states largely followed the internal administrative boundaries of the Soviet republics, though disputes remain over territories such as Crimea and the Donbas.

Yugoslavia’s dissolution was far more violent. The Badinter Arbitration Committee of the European Community provided legal opinions that recognized the republics’ borders as internationally protected, but this did not prevent conflicts in Croatia, Bosnia, and Kosovo. The Dayton Agreement (1995) ended the Bosnian War and established the internal borders of Bosnia and Herzegovina, creating two entities: the Federation of Bosnia and Herzegovina and the Republika Srpska. The Treaty of Osimo (1975) had already resolved border issues between Italy and Yugoslavia, and subsequent agreements in the early 2000s settled the status of Montenegro and Kosovo.

Contemporary Border Dynamics and Treaty Legacies

Today, European borders are defined by a complex patchwork of treaties, some centuries old and others quite recent. The European Union has fundamentally altered the significance of borders, creating a space where internal frontiers are largely open and subject to shared governance. The Schengen Agreement (1985) abolished border checks between participating states, representing a dramatic departure from the heavily fortified borders of the past. However, the external borders of the EU remain a source of tension and policy debate.

Several contemporary disputes trace their origins to historical treaties. The Cyprus dispute between Greek and Turkish Cypriots originates from the 1960 Treaties of Guarantee that established the Republic of Cyprus. The Northern Ireland protocol of the Brexit settlement directly references the Anglo-Irish Treaty of 1921. Russian territorial claims in Ukraine and Georgia draw on historical grievances related to treaties such as the Molotov-Ribbentrop Pact and the postwar settlement of 1945. The Schengen area itself is legally based on a treaty that supersedes earlier bilateral agreements, creating new forms of cooperation that transcend traditional state boundaries.

Treaties remain the primary legal mechanism for resolving border disputes peacefully. The Permanent Court of Arbitration and the International Court of Justice have adjudicated numerous European border cases based on the interpretation of historical treaties. The 2009 decision in the Maritime Delimitation in the Black Sea case between Romania and Ukraine, for example, relied on the interpretation of post-World War II treaties and subsequent agreements. Similarly, the Arbitration Commission of the Peace Conference on Yugoslavia provided crucial legal guidance during the dissolution of that federation.

Conclusion

The borders of modern Europe are the accumulated product of centuries of treaty-making. From the Peace of Westphalia to the recent agreements governing European Union expansion, treaties have provided the legal framework for transferring territory, establishing sovereignty, and managing conflict. While many borders are now stable and widely recognized, the legacy of historical treaties continues to shape political identities, economic relationships, and regional disputes.

Treaties are not merely documents of their time; they create durable realities that influence generations. The redrawing of borders after World War I created new states but also fostered resentment and instability that contributed to World War II. The postwar settlement froze borders for half a century, only to see them transform again with the end of the Cold War. Understanding this historical arc is essential for navigating contemporary debates about sovereignty, self-determination, and European integration.

The continued relevance of treaties in European border management is evident in the work of the Venice Commission, the OSCE, and other European institutions that monitor treaty compliance and facilitate negotiations. The Council of Europe provides a legal framework for treaty-based cooperation that spans the continent. The European Commission handles cross-border cooperation programs that help mitigate the practical effects of historical borders.

As Europe faces new challenges from migration, climate change, and geopolitical competition, the treaty system that has shaped its borders for centuries will continue to evolve. The principles of sovereignty, consent, and peaceful change that underpin modern treaty law offer the best hope for managing future border adjustments without the violence that characterized much of the continent’s history. The story of Europe’s borders is ultimately the story of how people have sought to order their political space through agreement, creating a legal legacy that remains foundational to the continent’s identity.