Countries in Northern Europe were to have a marked impact on the Thirty Years War.
The main countries involved in this region were Denmark and Sweden.
After 1523, Denmark continually tried to get Sweden back into the Kingdom of Denmark-Norway. She failed to do this and lost a number of Baltic outposts in the process of attempting to re-conquer Sweden. However, Sweden itself was going through a dynastic struggle.
In 1587, Sigismund, son of John III of Sweden, was elected king of Poland.
In 1592, John III died and Sigismund was declared king of Sweden as well as being king of Poland.
Sigismund proved to be an unpopular king in Sweden and the brother of John III, Duke Charles, forced Sigismund to return to Poland in 1593. Charles became the effective ruler of Sweden and was crowned king Charles IX in 1604. Between 11606 and 1609, Poland was affected by the Rokosz rebellion and Sigismund had to deal with this. Therefore, he did not have the opportunity to challenge Charles over the Swedish throne.
In 1611, Denmark attacked Sweden. The Danish leader, Christian IV, was angered at the increasing prosperity of Sweden and her rapidly developing economy which would clearly challenge Denmark in future years. Charles IX died during the war and in 1611 he was succeeded by his son Gustavus Adolphus (also known as Gustav II Adolf). Aided by his chancellor, Axel Oxenstierna, he managed to end the Danish War by the Peace of Knarad in 1613. The Danes received a substantial sum of cash in return for Alvsborg, Sweden’s only port on the North Sea. However, despite this blow to Sweden’s financial reserves, Gustavus had got Sweden out of a war that was draining her economy anyway.
The dynastic feud that had characterised Sweden before the Danish War continued after it. Gustavus was forced to seek closer ties with Russia which meant that Poland was faced with potential enemies on both sides of her borders. Charles IX had already started the process of befriending Russia when he had given the tsar, Boris Gudunov, military aid in a war between Russia and Poland.
Boris Gudunov himself faced problems among the aristocracy of Russia. The huge authority tsars such as Peter the Great had, was not true for many Russian tsars who lead Russia in name but had relatively little power outside of Moscow. A link up with Sweden would advance his power and so some Russian aristocrats supported the claim of Wladislaw to be tsar of Russia. Wladislaw was the son of Sigismund of Poland. Boris was overthrown and Wladislaw became tsar of Russia. Sigismund had now ended the fear of an enemy on two borders but his son proved an unpopular ruler and in 1613 he was in turn overthrown by Michael Romanov. He asked Sweden for help as he feared a Polish invasion but Gustavus refused as he believed that it might provoke a Polish reaction against Sweden.
In 1614, Sweden allied with Holland (both were protestant and had growing economies) and in 1615, Sweden allied with the Evangelical Union of Germany - a collection of Protestant German states. With this behind him, Gustavus used his influence and the threat of military backing from his new allies, to force Russia and Poland into a truce at the Treaty of Duelmo signed in 1618.
Sweden and Poland came to a truce in 1618 with the Treaty of Tolsburg. By 1618, Gustavus had developed a reputation as a skilled diplomat and by 1620, Sweden was considered a major European power.
The one destabilising factor in the region was Sigismund. He still considered Sweden to be his and his love of Catholicism clashed with a state - Sweden - that had outlawed Catholicism. Sigismund was nick-named "Pope Phillip II" because of his zeal in converting Polish Protestants to Catholicism. Sigismund was a fervent supporter of the Counter-Reformation and Poland was known as the "Spain of the North". Sigismund had the potential to destabilise the whole region especially as Sweden was seen as a bastion of Protestantism.