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Margaret I, daughter of Valdemar IV (Valdemar Atterdag), married Håkon VI of Norway as part of a dynastic effort to unite Denmark, Norway, and Sweden, since Håkon was related to the Swedish royal family. Their son, Olaf II, was intended to rule the three kingdoms, but following his early death in 1387, Margaret assumed control. Ruling from 1387 until her death in 1412, she skillfully unified Denmark, Norway, and Sweden, along with territories like the Faroe Islands, Iceland, Greenland, and parts of modern-day Finland, under her leadership. This unification led to the formation of the Kalmar Union, officially established in 1397, representing the first major consolidation of the Nordic countries under a single monarch.
After Margaret's death, her great-nephew Eric of Pomerania became King of Denmark in 1412. However, Eric lacked Margaret's political skill and soon embroiled Denmark in conflicts with the Holstein counts and the city of Lübeck. These tensions prompted the Hanseatic League to impose a trade embargo on Scandinavia, which severely impacted Sweden's economy and led to a revolt. By 1439, all three kingdoms of the Kalmar Union had deposed Eric, signaling the beginning of the union's fragmentation.
Despite these challenges, the vision of Scandinavian unity endured. In 1440, Eric's nephew, Christopher of Bavaria, was elected king of Denmark, Norway, and Sweden, briefly reuniting the kingdoms from 1442 to 1448. However, the union was fragile, as Swedish resentment towards Danish rule turned the Kalmar Union into a largely symbolic entity. During the reigns of Christian I (1450–1481) and his son Hans (1481–1513), tensions between Denmark and Sweden continued, leading to several conflicts.
In the early 16th century, Christian II ascended the throne in 1513 and attempted to reassert control over Sweden. After capturing Stockholm in 1520, he ordered the execution of around 100 leaders of the Swedish anti-unionist movement, an event known as the Stockholm Bloodbath. This act eliminated any hope of preserving the Kalmar Union, sparking outrage across Sweden and leading to its final break from Danish control, effectively ending the union and the dream of a unified Scandinavian kingdom.