John III of Sweden in the context of "Eric XIV of Sweden"

Sigismund III Vasa (Polish: Zygmunt III Waza, Lithuanian: Žygimantas Vaza; 20 June 1566 – 30 April 1632N.S.) was King of Poland and Grand Duke of Lithuania from 1587 to 1632 and, as Sigismund, King of Sweden from 1592 to 1599. He was the first Polish sovereign from the House of Vasa. Religiously zealous, he imposed Catholicism across the vast realm, and his crusades against neighbouring states marked Poland's largest territorial expansion. As an enlightened despot, he presided over an era of prosperity and achievement, further distinguished by the transfer of the country's capital from Kraków to Warsaw.

Sigismund was the son of King John III of Sweden and his first wife, Catherine Jagiellon, daughter of King Sigismund I of Poland. Elected monarch of the Polish–Lithuanian Commonwealth in 1587, he sought to unify Poland and Sweden under one Catholic kingdom, and when he succeeded his deceased father in 1592 the Polish–Swedish union was created. Opposition in Protestant Sweden caused a war against Sigismund headed by Sigismund's uncle Charles IX, who deposed him in 1599.

Catherine Jagiellon (Polish: Katarzyna Jagiellonka; Swedish: Katarina Jagellonica, Lithuanian: Kotryna Jogailaitė; 1 November 1526 – 16 September 1583) was a princess of the Polish–Lithuanian Commonwealth and Queen of Sweden from 1569 as the wife of King John III. Catherine had significant influence over state affairs during the reign of her spouse. She negotiated with the pope to introduce Counter-Reformation in Sweden. She was the mother of Sigismund III Vasa.

The Polish–Swedish union was a short-lived personal union between the Polish–Lithuanian Commonwealth and the Kingdom of Sweden between 1592 and 1599. It began when Sigismund III Vasa, elected King of Poland and Grand Duke of Lithuania, was crowned King of Sweden following the death of his father John III. The union ended following a civil war in Sweden in which he lost the crown to his uncle, who eventually became Charles IX. Sigismund afterwards returned to Warsaw and pursued a war against his former realm.

Charles IX, also Carl (Swedish: Karl IX; 4 October 1550 – 30 October 1611), reigned as King of Sweden from 1604 until 1611. He was the youngest son of King Gustav I (r. 1523–1560) and of his second wife, Margaret Leijonhufvud, the brother of King Eric XIV and of King John III, and the uncle of Sigismund, who became king both of Sweden and of Poland. By his father's will Charles received, by way of appanage, the Duchy of Södermanland, which included the provinces of Närke and Värmland; but he did not come into actual possession of them till after the fall of Eric and the succession to the throne of John in 1569.

Both Charles and one of his predecessors, Eric XIV (r. 1560–1569), took their regnal numbers according to a fictitious history of Sweden. He was actually the third Swedish king called Charles.

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.

The Reformation in Sweden is generally regarded as having begun in 1527 during the reign of King Gustav I of Sweden, but the process was slow and was not definitively decided until the Uppsala Synod of 1593, in the wake of an attempted Counter-Reformation during the reign of John III (1568–1592).

The Swedish Reformation caused the Church of Sweden to break with the Roman Catholic Church. It is considered to mark the end of the Swedish Middle Ages. The Reformation resulted in both Sweden and Finland becoming Protestant countries, as the latter formed an integral part of Sweden at the time.