Charles X Gustav suddenly dies of an illness in early 1660, while planning an invasion of Norway.

Following his death, Sweden makes peace in the Treaty of Copenhagen.

The Swedes return Trøndelag to Norway and Bornholm to Denmark, but keep both Bahusia and Terra Scania.

The Netherlands and other European powers accept the settlement, not wanting both coasts of the Sound controlled by Denmark.

This treaty establishes the boundaries between Norway, Denmark, and Sweden that still exist today.

During the Second Northern War, Denmark–Norway launches a war of revenge in 1657 against Sweden (at this time distracted in Poland), which turns into a complete disaster.

The war becomes a disaster for two reasons: Primarily, because Denmark's new powerful ally, the Netherlands, remains neutral as Denmark is the aggressor and Sweden the defender.

Secondly, the Belts freeze over in a rare occurrence during the winter of 1657-1658, allowing Charles X Gustav of Sweden to lead his armies across the ice to invade Zealand.

In the following Treaty of Roskilde, Denmark–Norway capitulates and gives up all of Eastern Denmark (Danish: Skåne, Halland, Blekinge and Bornholm), in addition to the counties of Bahusia (Norwegian: Båhuslen) and Trøndelag in Norway.

Holstein-Gottorp is also tied to Sweden, providing a gateway for future invasions from the south, but the Second Northern War is not yet over.

Charles X Gustav of Sweden holds a council of war, three months after the peace treaty with Denmark had been signed, in which he decides to simply wipe Denmark from the map and unite all of Scandinavia under his rule.

Once again, the Swedish army arrives outside Copenhagen.

However, this time the Danes did not panic or surrender.

Instead, they decide to fight and prepare to defend Copenhagen.

Frederick III of Denmark has stayed in his capital and now encourages the citizens of Copenhagen to resist the Swedes, by saying he will die in his nest.

Furthermore, this unprovoked declaration of war by Sweden finally triggers the alliance that Denmark–Norway has with the Netherlands.

A powerful Dutch fleet is sent to Copenhagen with vital supplies and reinforcements, which saves the city from being captured during the Swedish attack.

Furthermore, Brandenburg-Prussia, the Polish–Lithuanian Commonwealth and the Habsburg monarchy have gathered large forces to aid Denmark–Norway and fighting continues into 1659.

Sweden is the third-largest country in Europe by land area in the middle of the seventeenth century, only surpassed by Russia and Spain.

Sweden reaches its largest territorial extent under the rule of Charles X after the treaty of Roskilde in 1658.

The foundation of Sweden's success during this period is credited to Gustav I's major changes to the Swedish economy in the sixteenth century, and his introduction of Protestantism.

In the seventeenth century, Sweden is engaged in many wars, for example with the Polish–Lithuanian Commonwealth, with both sides competing for territories of today's Baltic states, with the disastrous Battle of Kircholm being one of the highlights.

The Swedes conduct a series of invasions into the Polish–Lithuanian Commonwealth, known as the Deluge.

Poland-Lithuania escapes the ravages of the Thirty Years' War, which ends in 1648, but the ensuing two decades subject the country to one of its severest trials.

This colorful but ruinous interval, the stuff of legend and the popular historical novels of Nobel laureate Henryk Sienkiewicz (1846-1916), become known as the potop, or deluge, for the magnitude of its hardships.

The emergency begins with an uprising of Ukrainian Cossacks that persists in spite of Warsaw's efforts to subdue it by force.

After the rebels win the intervention of Muscovy on their behalf, Tsar Alexis conquers most of the eastern half of the country by 1655.

Taking advantage of Poland's preoccupation, Charles X of Sweden rapidly overruns much of the remaining territory of the commonwealth in 1655.

Pushed to the brink of dissolution, Poland-Lithuania rallies to recover most of its losses to the Swedes.

Swedish brutality raises widespread revolts against Charles, whom the Polish nobles have recognized as their ruler in the meantime.

Under Stefan Czarniecki, the Poles and Lithuanians drive the Swedes from their territory by 1657.

Further complicated by noble dissension and wars with the Ottoman Turks, the thirteen-year struggle over control of Ukraine ends in the Truce of Andrusovo in 1667.

Although Russia had been defeated by a new Polish-Ukrainian alliance in 1662, Russia gains eastern Ukraine in the peace treaty.

Northeast Europe (1648–1659 CE): Treaty of Westphalia, Swedish Expansion, and Regional Rivalries

Between 1648 and 1659 CE, Northeast Europe experienced profound changes driven by the aftermath of the Thirty Years' War and the Treaty of Westphalia. The era was characterized by continued Swedish territorial expansion, renewed Danish–Swedish rivalry, and complex diplomatic maneuverings that reshaped regional alliances and cultural developments.

Impact of the Treaty of Westphalia

The Treaty of Westphalia (1648), ending the Thirty Years' War, confirmed Sweden's status as a major European power. Sweden secured significant territories along the Baltic coast, including Western Pomerania, reinforcing its dominance in the region and bolstering its economic and strategic influence. This settlement marked a critical turning point, enhancing Swedish prestige and geopolitical power.

Continued Swedish Expansion and Influence

Under Queen Christina until her abdication in 1654, and subsequently under Charles X Gustav (r. 1654–1660), Sweden consolidated territorial acquisitions in Livonia, Estonia, and along the Baltic coastline. Cities such as Reval (Tallinn), Riga, and Narva saw enhanced military fortifications and expanded governance, solidifying Swedish authority and economic prosperity.

Danish–Swedish Rivalry and Conflicts

Renewed hostilities emerged between Sweden and Denmark–Norway, culminating in the Northern Wars. Under King Frederick III (from 1648), Denmark–Norway challenged Sweden's regional ascendancy, leading to the outbreak of war in 1657. Despite initial Danish successes, Sweden achieved significant victories, notably crossing the frozen Danish straits in 1658 and forcing Denmark into the Treaty of Roskilde (1658), transferring substantial territories, including Scania, Halland, and Blekinge, to Sweden.

Polish–Lithuanian Commonwealth and Eastern Stability

The Polish–Lithuanian Commonwealth under King John II Casimir Vasa (from 1648) faced significant internal strife and external threats, notably from Sweden and Muscovy. The devastating Swedish invasion during the Deluge (1655–1660) severely weakened the Commonwealth, prompting diplomatic and military efforts to stabilize eastern territories and recover lost influence.

Muscovy’s Assertiveness and Expansion

Under Tsar Alexis I (r. 1645–1676), Muscovy intensified efforts to reclaim influence in the Baltic and neighboring regions. Alexis pursued active diplomatic and military strategies, challenging Polish–Lithuanian and Swedish interests, especially in contested territories like Livonia and Ingria, marking a more assertive Muscovite presence in regional politics.

Prussian Strength and Strategic Neutrality

The Duchy of Prussia, under Duke Frederick William I (the Great Elector), continued strengthening internal stability and economic prosperity. Prussian neutrality during regional conflicts enabled Königsberg and surrounding urban centers to flourish economically and politically, establishing Prussia as a significant and stable entity within Northeast Europe.

Economic Prosperity in Key Urban Centers

Cities such as Reval (Tallinn), Riga, Königsberg, and Visby remained economic hubs, driven by robust maritime commerce, merchant activities, and effective urban governance. These urban centers maintained regional economic stability despite military conflicts and political turbulence.

Cultural and Intellectual Vibrancy

Protestant institutions, particularly Lutheran universities and academies, continued to thrive, advancing educational and cultural development. Increased literacy, intellectual exchanges, and scholarly pursuits significantly enhanced Northeast Europe's cultural and intellectual landscape, reaffirming its position as a leading intellectual hub.

Scientific and Scholarly Advances

The region maintained a strong intellectual tradition, with continued contributions in astronomy, mathematics, and natural sciences. Inspired by scholars like Tycho Brahe, Northeast European scientists and intellectuals contributed significantly to European scientific discourse, fostering innovation and scholarly prestige.

Complex Diplomatic Maneuvers

Diplomatic interactions intensified during this period, marked by shifting alliances, strategic negotiations, and careful management of territorial interests. Relations among Sweden, Denmark–Norway, Poland–Lithuania, Muscovy, and Prussia remained fluid and strategically complex, reflecting ongoing geopolitical uncertainties.

Legacy of the Era

The period from 1648 to 1659 CE significantly reshaped Northeast Europe's political, territorial, and cultural landscapes. The aftermath of Westphalia, renewed rivalries, and substantial cultural advancements deeply influenced subsequent historical trajectories, setting enduring regional boundaries, political alliances, and cultural identities.

The Crown of Sweden is hereditary in the family of Vasa, and from Charles IX's time excludes those Vasa princes who had been traitors or descended from deposed monarchs.

Gustav Adolf's younger brother had died years earlier, and therefore there are only females left.

Despite the fact that there were living female lines descended from elder sons of Gustav I Vasa, Christina had been the heiress presumptive.

Although she is often called "queen", her father had brought her up as a prince and her official title is King.

As ruler, Christina had resisted demands from the other estates (clergy, burgesses and peasants) in the Riksdag of the Estates of 1650 for the reduction of tax-exempt noble landholdings.

Several princes of Europe aspire to her hand; but she has rejected them all.

Now twenty-eight, Christina knows it is expected of her to provide an heir to the Swedish throne.

Her first cousin Charles, the son of John Casimir, Count Palatine of Zweibrücken-Kleeburg and Catharina of Sweden, had at an early age become infatuated with her, and they had become secretly engaged before he left in 1642 to do army service for three years in Germany.

However Christina reveals in her autobiography that she felt "an insurmountable distaste for marriage"; likewise "an insurmountable distaste for all the things that females talked about and did".

She sleeps for three to four hours a night and is chiefly occupied with her studies; she forgets to comb her hair, dons her clothes in a hurry and uses men's shoes for the sake of convenience.

However she is said to possess charm, and the unruly hair becomes her.

Christina on February 26, 1649, had made public that she had decided not to marry, but wanted her first cousin Charles as heir to the throne.

The nobility objects to this, but the three other estates—clergy, burghers and peasants—accept it.

The coronation takes place in October 1650.

Christina goes to the castle of Jacobsdal, today known as Ulriksdal, where she enters a coronation carriage drawn with black velvet embroidered in gold, and pulled by six white horses.

The procession to Storkyrkan in Stockholm is so long that when the first carriages arrive at Storkyrkan, the last ones have not yet left Jacobsdal.

All four estates are invited to dine at the castle.

Fountains at the market place splash out wine, roast is served, and illuminations sparkle.

The participants are dressed up in fantastic costumes, like at a carnival.

The future King Charles X Gustav of Sweden, raised in the Swedish court alongside Queen Christina, had received an excellent civil education, and later learned the art of war under Lennart Torstenson, being present at the second Battle of Breitenfeld (1642) and at Jankowitz (1645).

He had from 1646 to 1648 frequented the Swedish court, supposedly as a prospective husband of his cousin the queen regnant, but her insurmountable objection to wedlock had put an end to these anticipations, and to compensate her cousin for a broken half-promise she had declared him her successor in 1649, despite the opposition of the Privy Council headed by Axel Oxenstierna.

Charles had in 1648 gained the appointment of commander of the Swedish forces in Germany.

The conclusion of the treaties of Westphalia in October of that year had prevented him from winning the military laurels he is said to have desired, but as the Swedish plenipotentiary at the executive congress of Nuremberg, he had had an opportunity to learn diplomacy, a science he is described as having quickly mastered.

As the recognized heir to the throne, his position on his return to Sweden was dangerous because of the growing discontent with the queen.

He had therefore withdrawn to the isle of Öland until June 5, 1654, when the abdication of Christina calls him to the throne.

Christina, now the former reigning queen of a Protestant nation, secretly converts to Catholicism.

The beginning of Charles X's reign is concentrated on the healing of domestic discords and on the rallying of all the forces of the nation round his standard for a new policy of conquest.

He contracts a political marriage on October 24, 1654 with Hedwig Eleonora, the daughter of Frederick III, Duke of Holstein-Gottorp, by way of securing a future ally against Denmark.

The combined forces of Khmelnitsky and the Muscovite boyar Buturlin strike against Volhynia simultaneous with the Russian invasion of Polish Livonia.