Stockholm Bloodbath in the context of Christian II of Denmark, Norway and Sweden

Gustav Eriksson Vasa (12 May 1496 – 29 September 1560), also known as Gustav I, was King of Sweden from 1523 until his death in 1560. He was previously self-recognised Protector of the Realm (Riksföreståndare) from 1521, during the Swedish War of Liberation against King Christian II of Denmark, Norway and Sweden. Gustav rose to lead this war following the Stockholm Bloodbath, where his father was executed. Gustav's election as king on 6 June 1523 (the future National Day of Sweden) and his triumphant entry into Stockholm eleven days later marked Sweden's final secession from the Kalmar Union.

During his reign, Gustav initiated the Protestant reformation in Sweden, transformed the country from an elective to a hereditary monarchy and established a standing army and navy.

The House of Vasa or Wasa was a royal house that was founded in 1523 in Sweden. Its members ruled the Kingdom of Sweden from 1523 to 1654 and the Polish–Lithuanian Commonwealth from 1587 to 1668. Its agnatic line became extinct with the death of King John II Casimir Vasa in 1672.

The Vasa dynasty descended from a 14th-century Swedish noble family, tracing agnatic kinship to Nils Kettilsson (Vasa) (d. 1378), the fogde of Tre Kronor Castle in Stockholm. Several members held high offices during the 15th century. In 1523, after the Stockholm bloodbath and the abolition of the Kalmar Union, Gustav Eriksson (Vasa) became King Gustav I of Sweden and the royal house was founded. His reign is sometimes referred to as the beginning of the modern Swedish state, which included the King's break with the Catholic Church during the Protestant Reformation and the foundation of the Church of Sweden.

The early Vasa era is a period in Swedish history that lasted between 1523–1611. It began with the reconquest of Stockholm by Gustav Vasa and his men in 1523, which was triggered by the event known as the Stockholm Bloodbath in 1520, and then was followed up by Sweden's secession from the Kalmar Union, and continued with the reign of Gustav's sons Eric XIV, John III, John's son Sigismund, and finally Gustav's youngest son Charles IX. The era was followed by a period commonly referred to as the Swedish Empire, or Stormaktstiden in Swedish, which means "Era Of Great Power".

Gustav's reign was marked by internal political and religious reforms, including the Protestant Reformation, where he converted to Protestantism and seized Catholic Church property and wealth, and unification of the provinces. At the death of Gustav in 1560, he was succeeded by his eldest son Eric. Eric was intelligent and skilled, but was in a constant strain with his brother and other noblemen. He engaged in warfare against both Denmark, Russia and Poland, but suffered periods of insanity in 1567. In 1568 he was dethroned and succeeded by his brother John.

Christian II (1 July 1481 – 25 January 1559), a monarch under the Kalmar Union, reigned as King of Denmark and Norway from 1513 until 1523. He was briefly King of Sweden from 1520 until 1521. As king of Denmark and Norway, he was concurrently Duke of Schleswig and Holstein in joint rule with his uncle Frederick.

As king, Christian tried to maintain the Kalmar Union between the Scandinavian countries which brought him to war with Sweden, lasting between 1518 and 1523. Though he captured the country in 1520, the subsequent slaughter of leading Swedish nobility, churchmen, and others, known as the Stockholm Bloodbath, caused the Swedes to rise against his rule. He was deposed in a rebellion led by the nobleman and later king of Sweden Gustav Vasa. He attempted to bring in a radical reform of the Danish state in 1521–22, which would have strengthened the rights of commoners at the expense of the nobles and clergy. The nobility rose against him in 1523, and he was exiled to the Netherlands, ceding the Danish throne to his uncle Frederick. After attempting to reclaim the thrones in 1531, he was arrested and held in captivity for the rest of his life, first in Sønderborg Castle and later at Kalundborg Castle. Supporters tried to restore him to power both during his exile and his imprisonment but they were defeated decisively during the Count's Feud in 1536. Christian died at Kalundborg in 1559.