The war took place in 1563–1570 between Sweden and united forces from Denmark and the German city of Lübeck. The two sides were fighting to gain control over the profitable Baltic Sea trade. The Danes also had a desire to re-create the Kalmar Union, a union that had joined the three kingdoms of Denmark, Sweden and Norway under one monarch. The war was fought largely at sea. Several battles were fought, of which the two biggest were those at Öland in May 1564 and at Bornholm in July 1565.
At the Battle of Öland, 25 Danish-Lübeckian ships with about 6,300 men onboard and 16 Swedish ships with 5,000 men onboard took part. The Danes’ advantage in numbers was partly offset by the Swedes’ more efficient artillery. Both sides suffered losses, with the Swedes losing their largest and fiercest warship, the Mars. A cannon exploded and the ship caught on fire. Many members of the Lübeckian crew had boarded the Mars to plunder it when the fire reached the gunpowder storage. Nearly 1,000 Swedes and Lübeckians died when the vessel sank.
At the Battle of Bornholm the two forces had more equal numbers of ships, and Sweden emerged victorious although at a high price. Three major Swedish ships were lost and close to 400 men perished. The Danes lost several ships and a great many seamen.
The largest single loss of ships in the Seven Years’ War occurred in 1566 – not in a naval battle, but in connection with a funeral. During a minor naval battle at the northern tip of Öland, a Danish nobleman was killed in battle. The Danish admiral wanted the nobleman’s funeral to take place on land, so the Danish-Lübeckian fleet anchored outside Visby on Gotland. But the wind changed direction and transformed into a storm that caused 11 Danish and 3 Lübeckian ships to capsize or smash against the rocks, killing about 1,500 men.
The Nordic Seven Years’ War had no clear winners – both sides suffered heavy losses in terms of both fleet and crew. In 1570, a peace treaty was signed in Stettin (what is today in Poland) after six months of negotiations. As part of the treaty, Sweden had to pay a ransom to recover the fortress of Älvsborg.