Casus Belli
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.
Shortly after his coronation in 1559, King Frederick II of Denmark ordered his ageing field-commander Johan Rantzau to avenge the humiliating Danish defeat against the small peasant republic of Ditmarsh 1500, which was defeated in a matter of a few weeks and brought under the Danish-Norwegian crown. During the next year, the Danish expansion continued and possessed the baltic island of Ösel.
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. During this conflict, King Eric of Sweden successfully obstructed the Danish plans to conquer Estonia. 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; a bone of contention since Christian III and Gustav Vasa. In February 1563, Swedish messengers were sent to Hesse to negotiate Eric's marriage with Christine of Hesse, but were held back in Copenhagen. In retaliation, Eric added the insignia of Norway and Denmark to his own coat of arms, and refused Danish requests to remove these symbols.
Lübeck, upset over obstacles of trade that Eric had introduced to hinder the Russian trade and withdrawn trade privileges, joined Denmark in a war alliance. The Polish–Lithuanian union also joined, desiring control of the Baltic trade. Skirmishes broke out in May 1563, before war was officially declared in August that year.
Read more about this topic: Northern Seven Years' War