During the Thirty Years' War (1618–1648), Finland, as part of the Kingdom of Sweden, played a significant role in Sweden's military campaigns. The war, which began as a conflict over religion and power in Central Europe, soon saw Sweden emerge as a major player under King Gustavus Adolphus. Finland’s contribution to the war effort came mainly through the recruitment of Finnish soldiers, including the famous "Hakkapeliitta" cavalry, known for their fierce and effective combat tactics.
Thousands of Finnish soldiers were conscripted into Sweden's army, many of whom fought in key battles in Germany and other parts of Europe. The war was brutal, and the toll on Finnish society was high. Families lost many men, and the countryside faced economic strain due to the constant demand for recruits and resources. However, Sweden's military successes during the war, largely supported by Finnish troops, helped the kingdom expand its influence across Europe.
The end of the war in 1648 with the Peace of Westphalia saw Sweden solidify its position as a major European power. For Finland, while it remained under Swedish control, the war left lasting scars in terms of population loss and economic hardship, but it also tied the region more closely to Sweden’s growing dominance in Europe.