The Russo-Swedish War of 1741–1743, also known as the War of the Hats, was a devastating conflict for Finland, then part of the Kingdom of Sweden. The war was instigated by Sweden’s "Hats" political party, which aimed to recover territories lost to Russia during the Great Northern War. However, poor preparation and internal discord led to a disastrous outcome for Sweden and Finland.
Finland became a battleground during the conflict, with Russian forces invading from Viborg in 1741 and winning decisive victories, including the Battle of Villmanstrand. Sweden's uncoordinated defense efforts, weakened by plague and poor leadership, were no match for the advancing Russian army. By 1742, Finland was fully occupied by Russian forces, a period known as the *Lesser Wrath* (*Pikkuviha*), which caused widespread suffering among the Finnish population.
The war ended with the Treaty of Åbo in 1743, where Sweden was forced to cede southeastern Finland, including Lappeenranta and Hamina, to Russia. This further shifted the border between Sweden and Russia along the Kymi River, continuing Finland’s partition and diminishing Sweden’s influence in the region. Finland's fate as a contested territory between Sweden and Russia was solidified, and the ceded lands later became part of the Russian Grand Duchy of Finland in 1812.