The Kalmar Union of the three former Scandinavian Kingdoms of Sweden, Norway, and Denmark lasted on and off from 1397 to 1523, until it finally collapsed following the continued Swedish resentment of Danish domination.

[7] A successful rebellion in 1471 led to Swedish victory at the Battle of Brunkeberg, which established a powerful anti-Union movement under the leadership of the Bonde–Sture nobles.

[8] More than 80 noble men and ladies, including leading citizens of Stockholm, were executed, but the result severely backfired on Christian II.

[7] The violence elicited strong reactions in Sweden for years to come,[8] and the Union was broken by the successful Swedish War of Liberation from 1521 to 1523.

The subsequent Danish kings Frederick I and Christian III, turned their attention mainly on the Reformation in Denmark–Norway and Holstein and the Count's Feud civil war, and relations with Sweden were generally peaceful.

[7] Gustav Vasa's Sweden was in a weak position in 1523, as access to the North Sea was dominated by the Danish Sound Dues and limited to a 20 kilometer stretch on the Kattegat in the vicinity of Älvsborg, close to modern Gothenburg.

In 1544 he used the old Scandinavian concept of Uppbåd (levy or the prerogative to call up some fraction of men from each district in an emergency) to establish one of the first native standing armies in Europe.

The men served on standby, remaining at home in peacetime, and being paid by tax concessions, but were required to assemble and drill.

After the deaths of Christian III and Gustav Vasa, in 1559 and 1560 respectively, both countries now had young and hawkish monarchs, Eric XIV of Sweden and Frederick II of Denmark.

Frederick II envisioned the resurrection of the Kalmar Union under Danish leadership, while Eric wanted to finally break the dominating position of Denmark.

[11] In 1561, when a sizeable remnant of the Order states in the northern Baltics were secularized by its grand master Gotthard Kettler, both Denmark and Sweden were attracted to intervene in the Livonian War.

He sought to dominate the Baltic Sea, while unsuccessfully pressing for Frederick to remove the traditionally Swedish insignia of Three Crowns from the Danish coat of arms;[13] a bone of contention since Christian III and Gustav Vasa.

In retaliation, Eric added the insignia of Norway and Denmark to his own coat of arms and refused Danish requests to remove these symbols.

At the beginning of the war the Danes advanced from Halland with a 25,000-strong army of professional mercenaries and captured Sweden's gateway to the west, Älvsborg Fortress, after only three days of bombardment and a six-hour assault on 4 September.

After the king's departure from his army, Charles de Mornay stepped in as the commanding officer and was beaten by the Danish at the Battle of Mared.

In 1564 the Swedes marched under Claude Collart[14] and re-occupied Jemtland, as well as Herjedalen and Trøndelag, including the city of Trondheim.

Initially facing little opposition from the locals, their subsequent ill treatment of the Trøndelag natives, along with tax pressure, laid the groundwork for later resistance to Swedish invasion.

Because only a fraction of the army would march, Denmark had to give up the plan to take the fortress of Kalmar and settle for an attack on Stockholm instead.

An attempt to cut off his retreat over the Holaveden failed, as Rantzius' army went over the ice of lake Sommen[16] and in the middle of February 1568 he returned to Halland.

Negotiators included the dukes of Pomerania, French messenger Charles Dancay, and Holy Roman Emperors Ferdinand I and Maximilian II.

[17] Enthroned as king with the name John III, he began negotiations with Denmark, and on 18 November the same year this led to a draft peace agreement in Roskilde.