The Russo-Swedish War of 1741–1743 (also known as The War of the Hats) was instigated by the Hats, a Swedish political party that aspired to regain the territories lost to Russia during the Great Northern War, and by French diplomacy, which sought to divert Russia's attention from supporting its long-standing ally the Habsburg monarchy in the War of the Austrian Succession. The war was a disaster for Sweden, which lost more territory to Russia.
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Finland was an integral part of Sweden from the Middle Ages until 1809. The starting point of Swedish rule is uncertain and controversial. It is traditionally linked to the First Swedish Crusade in the mid-12th century. Historical evidence of the establishment of Swedish rule in Finland exists from the middle of the 13th century onwards.
Swedish rule ended in 1721 in most of so-called Old Finland, the south-eastern part of the Finnish territories, as a result of the Great Northern War. Sweden ceded the remainder of Old Finland in 1743, following the Hats' War. Swedish rule over the rest of Finland ended on 17 September 1809, when the signing of the Treaty of Hamina ended the Finnish War. As a result, the eastern third of Sweden was ceded to the Russian Empire and became established as the autonomous Grand Duchy of Finland.