John, duke of Finland, seeks a foothold in the east.

Contrary to King Eric’s wishes, his half brother had signed a treaty in 1561 with Sigismund II, king of Poland, agreeing to marry the king's youngest daughter by Bona Sforza, Catherine Jagellon.

Married on October 4, 1562, in the Lower Castle of Vilnius, Lithuania, the newlyweds set up house in Turku Castle, establishing a Renaissance court here.

Duke John's dealings in Livonia cause Eric ultimately to declare war on his brother, sending ten thousand men to besiege the castle, which capitulates on August 12, 1563.

Catherine and her husband are taken to Sweden and imprisoned in Gripsholm Castle on charges of treason, while her unsuccessful suitor, Tsar Ivan, negotiates with Eric over Catherine, asking for her to be separated from John and sent to marry him in Russia.

This causes alarm to both Catherine and her relations.

The Swedish people are beginning to become dissatisfied with their monarch, who is becoming increasingly insane.

German royal emissaries have meanwhile been sent to negotiate a peace at the meeting place of Rostock, but no Swedes had appeared.

Emissaries from Denmark and Lübeck in Stockholm declare war on August 13, 1563.

Nobles of Lübeck, Holstein, and Schleswig join their troops to Frederick’s army at Halland in southwestern Sweden.

Frederick, advancing with a twenty-five thousand-strong army of professional mercenaries, lays siege to Älvsborg Fortress, Sweden's gateway to the west.

The Danes, having accomplished the fall of Älvsborg on September 4, after only three days of bombardment and a six-hour assault, have achieved their aim aim of cutting off Sweden from the North Sea, blocking the all-important salt imports.

Erik’s forces, moving into the Danish province of Skane in extreme southern Sweden, attack Halmstad without result, the Swedish counterattack being driven back by the professional Danish army.

The Swedish force, after the king's departure from his army, is beaten by the Danish at Mared.

A sea battle breaks out near Öland on September 11, whereupon the war becomes a stalemate that is to last until late in the following spring.