Denmark–Sweden relations

Medieval contacts and the Kalmar Union

Denmark and Sweden emerged as unified kingdoms in the context of the Christianization of Scandinavia in the 8th and 10th centuries respectively and interacted closely with one another through trade, dynastic ties and military conflicts. An early example of conflict was the conquest of Gotland in 1361. The Danish king Valdemar IV Atterdag landed with his troops on the Swedish-influenced island of Gotland and defeated a force of Gotlandic peasants. Gotland subsequently remained under Danish rule for almost 300 years—an event that was to burden relations between the two Kingdoms for a long time.^[2] Nevertheless, the Middle Ages also saw phases of largely peaceful coexistence and close dynastic interconnections, which soon paved the way for a personal union.

In 1397, the Kalmar Union was founded under Queen Margaret I, uniting the kingdoms of Denmark, Norway and Sweden under one monarch. The union also emerged as a response to pressure from the Hanseatic League and other powers in the Baltic Sea region.^[3] Although Denmark initially dominated this personal union because of its greater resources, the individual structures of the member states remained intact. Despite all differences, close cooperative relations existed in the Late Middle Ages, but these increasingly eroded during the 15th century.^[4] In Sweden, resistance arose against Danish predominance; Swedish regents such as members of the Sture family at times led the government and defended Sweden's autonomy. Repeated Union Wars (Unionskrigene) followed: between 1434 and 1523 there were several uprisings and wars in which Swedish nobles fought against the Danish kings.

The tensions culminated in the early 16th century. The Danish king Christian II attempted to subjugate Sweden by force. After the capture of Stockholm in 1520, he invited the Swedish aristocracy to his coronation festivities, but broke his promise of amnesty and had around 80–90 Swedish nobles and dignitaries executed in a massacre in November 1520, the so-called Stockholm Bloodbath.^[5] This event shocked Swedish society and reignited the uprising against Danish rule. The Swedish Privy Council proclaimed the nobleman Gustav Vasa leader of the struggle for liberation. By June 1523 he had captured Stockholm and was elected king of Sweden as Gustav I. Sweden thereby left the Kalmar Union. From then on, Sweden and Denmark once again existed as separate kingdoms, and their relations changed from union to bitter rivalry over territory in the Baltic Sea region.^[4]

Rivalry and wars in the early modern period (16th–18th centuries)

The tensions erupted in the Northern Seven Years' War (1563–1570). Denmark under King Frederick II sought to restore the union, while Sweden under Eric XIV and later Johan III defended its independence. Denmark prevailed in some battles, but the large and sparsely populated Swedish realm proved impossible to conquer permanently. In the Treaty of Stettin of 1570, Denmark finally recognized Swedish sovereignty. Sweden, for its part, had to pay a large sum to Denmark in order to recover the occupied Älvsborg Fortress (Göteborg's access to the North Sea). The fact that Sweden was able to retain the territories it had acquired in the Baltic region, especially Estonia, was a sign of its rise as a new Baltic power.^[6][7]

In the following decades the conflict subsided for a time, partly because Sweden was involved in other wars against Russia and Poland-Lithuania. The rivalry nevertheless persisted. At the beginning of the 17th century, Christian IV in Denmark and Gustavus Adolphus in Sweden were two ambitious monarchs whose reigns reignited the antagonism. Christian IV viewed Swedish expansion into Livonia with concern, particularly because it harmed Denmark economically, for example through a Swedish blockade of Livonian ports. In 1611, Christian IV used a dispute over trade routes in Finnmark and claims to Northern Norway as a pretext to declare war on Sweden. The Kalmar War (1611–1613), named after the city of Kalmar, which was captured by the Danes, again revealed the pattern of earlier conflicts. Denmark dominated at sea, while Sweden, despite defeats, could not be defeated on land. Christian IV managed to seize the border fortresses of Kalmar and Älvsborg, forcing Sweden to make major concessions in the Treaty of Knäred in 1613. Sweden had to renounce its claims in Northern Norway and pay the Danish crown an enormous contribution of one million rixdalers in order to regain Älvsborg. This indemnity, the so-called Älvsborg ransom, made Denmark temporarily wealthy and strengthened Christian's position to such an extent that around 1620 he was regarded as one of the financially most powerful rulers in Europe. Denmark invested the money in expanding its navy and fortresses as well as in economic projects, all with the aim of securing Danish maritime supremacy and the Sound Dues.^[4] The Sound Dues, levied from 1429 on every ship passing through the Øresund strait between Denmark and Sweden, remained an important source of revenue and at the same time a constant point of contention: they symbolized Denmark's claim to control the "gateway to the Baltic Sea", much to the annoyance of Sweden and trading nations such as the Dutch Republic.^[8]

During the Thirty Years' War (1618–1648), Sweden finally rose to the status of a great power, while Denmark temporarily lost influence after its unsuccessful intervention (1625–1629). Sweden, led by Gustavus Adolphus, won significant victories in Central Europe and brought large parts of the southern Baltic coast under its control. This Swedish predominance in turn threatened Denmark. After Sweden had also established a presence in Northern Germany toward the end of the war, it attacked Denmark directly in 1643. Field Marshal Lennart Torstensson marched into Danish Holstein with experienced troops without warning. The Torstenson War (1643–1645) became a fierce feud between the old enemies: Christian IV stubbornly defended his realm—he himself lost an eye in the naval battle off the Colberger Heide—but Denmark ultimately succumbed to Swedish superiority on land and to the support of the Dutch, allied with Sweden, at sea.^[9] In the decisive Battle of Fehmarn Belt in October 1644, the Swedes and their allies sank several Danish warships; this victory marked the beginning of the end of Danish great-power status in the Baltic Sea region.^[8][4] In the Treaty of Brömsebro of 1645, Sweden imposed harsh terms: Denmark had to cede the Baltic island of Gotland as well as the formerly Norwegian-Danish provinces of Jämtland and Härjedalen to Sweden. Sweden also received the Danish duchy of Halland as a pledge for 30 years and was exempted from the Sound Dues.

As Sweden under King Charles X Gustav expanded still further after the Peace of Westphalia, including into Poland and the present-day Baltic states, the Danish king Frederick III sensed an opportunity for revenge and declared war on Sweden in 1657. This move proved disastrous: Charles X reacted swiftly, invaded Jutland with an army and in the winter of 1658 accomplished a military feat by marching his troops across the frozen Danish Belt straits from Jutland to the gates of Copenhagen.^[9] Denmark was defenseless. Frederick III was forced to accept enormous territorial losses in the humiliating Treaty of Roskilde of February 1658. Denmark had to cede all of Scania (Skåne), Blekinge and Halland—its three eastern provinces on the Scandinavian mainland—to Sweden, as well as the Baltic island of Bornholm and the formerly Norwegian territories of Bohuslän on the west coast and Trøndelag. These cessions meant that Sweden now controlled the entire southern tip of Scandinavia, revising the strategic situation in Northern Europe in Sweden's favor. Sweden had thereby massively expanded its territory at Denmark's expense and broken traditional Danish predominance in the Baltic region.^[10][11]

Hardly had the ink on the treaty dried when Charles X Gustav attempted in the summer of 1658 to conquer the rest of Denmark once and for all by besieging Copenhagen. This triggered a desperate Danish defense under King Frederick III. With support from the Dutch Republic, which sought to prevent a Swedish monopoly in the Baltic Sea, Copenhagen was successfully defended. The war ended in 1660 with the Treaty of Copenhagen, which forced Sweden to withdraw. Although Denmark's main losses from the Treaty of Roskilde remained—Scania and the other eastern Danish provinces stayed Swedish—Denmark regained Bornholm and the Trøndelag region in Norway, where uprisings against the Swedes had broken out. The borders between the two realms were thereby essentially fixed as they still run today between Denmark, including Norway until 1814, and Sweden. For Denmark, however, the loss of its former core lands remained a deep trauma. The formerly Danish province of Scania was intensively "Swedified" by Sweden in the following years, but the population continued to resist the new rule for some time. Denmark, for its part, did not immediately abandon hope of recovering Scania.^[12][11]

King Christian V of Denmark used Sweden's weakness during the Franco-Dutch War to make another attempt to recover the lost provinces in the Scanian War. Denmark achieved several military successes in this war: the Danes were able temporarily to occupy almost all of Scania, and in the Battle of Lund in 1676 and other engagements they confronted the Swedes directly. Parts of the local population welcomed the Danes, making Swedish rule more difficult. In the end, however, the European great powers, above all France, which supported Sweden, prevented any territorial revision: in the Treaty of Lund of 1679, Sweden recovered Scania and there were no border changes. The "Skåne question", meaning the Danish desire to regain Scania, remained emotionally present in the 18th century, but receded politically. The great maritime powers did not want one power alone to control the Øresund and therefore prevented Denmark from reincorporating Scania. Sweden subsequently began to integrate the formerly Danish territories more strongly, while a distinct regional identity between the nations gradually developed in Scania.^[12]

In the 18th century, direct Danish-Swedish confrontation initially subsided somewhat as other conflicts came to the fore. Sweden became involved in wars with Russia, which Denmark repeatedly tried to exploit. Thus, at the beginning of the Great Northern War (1700–1721), Denmark attacked Sweden in an attempt to make good old losses. Although Denmark suffered initial setbacks—the Swedish king Charles XII forced Denmark out of the war for a time in the Treaty of Traventhal in 1700—Denmark re-entered the conflict after Sweden's defeat at the Battle of Poltava in 1709. In the final phase of the war, Danish troops fought alongside Russia and Saxony against Sweden, which was ultimately defeated. In 1720, Denmark concluded the Treaty of Frederiksborg with exhausted Sweden. In it, Sweden did not have to cede territory to Denmark—Sweden retained Scania and all borders that had existed since 1660—but lost its former special status: Sweden was forced to give up its exemption from the Sound Dues and agreed to pay the Danish royal house a large sum of 600,000 rixdalers. In addition, Swedish support for the ducal collateral line of Holstein-Gottorp came to an end. This ducal family, allied with Sweden and ruling part of the Duchy of Schleswig, was disempowered by Denmark, and Schleswig was fully integrated into the Danish Realm in 1721.

In the second half of the 18th century, only minor direct conflicts occurred. One example was the so-called Theatre War of 1788–1789, during which Sweden under Gustav III actually attacked Russia, while Denmark, as Russia's ally, briefly became active in Norway against Sweden. This conflict ended quickly and without border changes, as Great Britain and Prussia pressed for peace. Overall, direct hostilities between Danes and Swedes receded into the background during this period. Both countries retained a degree of mistrust toward each other: in Denmark the memory of lost provinces lived on, while in Sweden Denmark was seen as a traditional adversary. Yet after 1721 there was rarely an immediate danger of war. The outcome of the Great Northern War had reduced both Denmark and Sweden to secondary powers in the Baltic region, which was now dominated by rising powers such as Prussia and Russia.

19th century: From conflict to cooperation

At the beginning of the 19th century, relations between Sweden and Denmark changed fundamentally. The last direct military confrontations between the two countries took place in the course of the Napoleonic Wars. From 1807, Denmark, as an ally of Napoleon, came into conflict with Sweden, which had sided with France's opponents. The Anglo-Swedish war against Denmark and the subsequent negotiations led to the decisive Treaty of Kiel of 14 January 1814: under pressure from the great powers, Denmark had to cede its centuries-old territory of Norway to Sweden. Norway, united with Denmark since 1380, was forced into a personal union with Sweden, thereby considerably expanding Swedish influence. In return, Denmark received the small territory of Swedish Pomerania as compensation, but this gain was minor and had to be ceded to Prussia in 1815. For Denmark, 1814 was a national catastrophe: after the loss of large parts of its fleet in 1807 and a state bankruptcy in 1813, the cession of Norway marked the final collapse of the old Danish-Norwegian composite state. Sweden, by contrast, had fulfilled its long-standing wish for territorial expansion at Denmark's expense, albeit at the price of its own losses, having lost Finland to Russia in 1809. With the Treaty of Kiel, the early modern rivalry had effectively been decided and the map of the North reorganized: two formerly great realms had been reduced in size, and the basis for the modern nation-states of Scandinavia had been laid.^[10]

After 1814, open hostilities between Sweden and Denmark ended. In the 19th century, both states developed domestically toward modern constitutional monarchies and industrial states, largely without mutual conflicts. Both countries finally reoriented themselves after the end of their great-power status. During this period, the movement of Scandinavism emerged: intellectuals and liberal circles in Denmark, Sweden and Norway, which was united with Sweden, began in the 1830s to advocate closer cooperation or even unity among the Scandinavian peoples. They invoked shared culture and history in order to create a "brotherhood" of the three kingdoms in an age of nationalism. This idea gained particular support among students and the bourgeoisie in the 1840s and 1850s. With the death of the old rivals Christian VIII of Denmark in 1848 and Charles XIV John of Sweden in 1844, the path seemed open for reconciliation. Charles XV, king of Sweden and Norway from 1859, showed open solidarity with Denmark: when Denmark became involved in a conflict with Prussia and Austria over Schleswig-Holstein in 1863–1864, Charles XV had initially even promised military assistance.^[13]

The political reality, however, proved sobering. When the Second Schleswig War broke out in 1864, the Swedish government hesitated. Domestic resistance in Stockholm and the fear of standing alone against Prussia and Austria led Sweden to withdraw its promise. After the Swedish retreat, Denmark, which had relied on an alliance until the end, ultimately faced the war without the expected support of its northern neighbor. The Danish defeat of 1864, and the loss of Schleswig and Holstein to Prussia, deeply disappointed the Scandinavist movement and effectively marked its end.^[13] Nevertheless, the shared shock over German power in Denmark and Sweden led in the long term to a more realistic but still friendly relationship. Both countries turned to domestic reforms and cooperated from then on without a formal alliance. In 1873, the two monarchies, together with Norway, founded the Scandinavian Monetary Union, which lasted until 1914—a sign of economic rapprochement.

20th and 21st centuries: World wars and Scandinavian integration

During the First World War (1914–1918), both countries remained neutral, as did Norway, which had become independent in 1905. In order to coordinate and safeguard this policy of neutrality, the kings of the three Scandinavian states met in Malmö in December 1914. There, Sweden, Denmark and Norway demonstratively declared their common intention to remain neutral in the war. This "Declaration of the Three Kings" and further agreements helped ensure that the world war left Northern Europe largely untouched.^[14] Denmark and Sweden developed into stable democracies during the interwar period and remained so during the Great Depression. In the Second World War (1939–1945), neutral Denmark was invaded by Nazi Germany in 1940. Sweden remained officially neutral and, under German pressure, had to permit transit traffic for the war against the Soviet Union. An outstanding example of Danish-Swedish cooperation in wartime was the rescue of the Danish Jews in October 1943: when the German occupation authorities began to deport Jewish citizens living in Denmark, the Danish resistance and civilian population set a spontaneous rescue operation in motion. Within a few days, more than 7,000 Danish Jews were secretly brought across the Øresund to Sweden and taken into safety there.^[15][16]

After the war, the division of Europe into power blocs solidified. Sweden and Denmark chose different security-policy approaches: Denmark became a founding member of NATO in 1949, while Sweden remained neutral and did not join a military alliance. Despite this difference, relations remained close and pragmatic. As early as 1952, Denmark, Sweden and Norway, later joined by Finland and Iceland, founded the Nordic Council, a forum for cooperation among parliaments and governments.^[17] An important step was the introduction of the Nordic Passport Union in 1954, which allowed citizens of Denmark, Sweden and the other Nordic countries to travel between the countries without passport controls. Together with the Benelux states, the Nordic countries were thus among the pioneers of European integration. Decades before the EU's Schengen Agreement was implemented, Scandinavia had effectively abolished internal borders.^[18] Diplomatically, Copenhagen and Stockholm consulted each other regularly; during the Cold War they often jointly advocated détente. Both were among the initiators of the CSCE in the 1970s and supported nuclear disarmament in Europe. Denmark joined the EEC in 1973, while Sweden joined the EU only after the end of the Cold War, in 1995. Both countries later decided against adopting the euro.^[19]

In the 21st century, relations between Denmark and Sweden are intensive and friendly. The Øresund fixed link, opened in July 2000, is symbolic of this: a 16-kilometre fixed bridge-tunnel connection across the Øresund has since linked the Danish capital region of Hovedstaden with the southern Swedish city of Malmö, in the once-contested region of Scania. The connection enabled the two regions to continue growing together into a cross-border metropolitan area.^[18] Since the beginning of the Russo-Ukrainian War, Sweden and Denmark have expanded their military cooperation in the Baltic Sea region out of concern over Russia and increased their military readiness. The Russian invasion of Ukraine in 2022 prompted Sweden to join NATO as its 32nd member in April 2024.^[20]