The Treaty of Nöteborg was signed by Sweden and Novgorod in 1323, splitting what is now Finland in half. The border also split Southern Savo in half. The exact location of the border is unclear as it was not marked on the ground, and thus Finns slowly started to settle on the Novgorod side of the border, too, and in practice, Sweden’s reign had also extended over the border set by the treaty by the end of the 15th century.

During the Great Northern War, Russia occupied all of Finland from 1713 to 1721; this period of time is known by the name the “Greater Wrath”. During the occupation, the Swedish and Russian armies robbed civilians, Finns were harshly taxed and up
to 10,000 Finns were shipped to Russia and forced to do slave labour.The majority of state officials and priests fled to Sweden.

Once again, Southern Savo became a theatre of war in the Russo-Swedish War from 1741 to 1743. The war resulted in a new border between Sweden and Russia being drawn in 1743; this border split Southern Savo in half. This was not, however, a completely negative matter for the people living in the region as doing business and smuggling goods over the border was profitable at times.

In 1788, war broke out once again between Sweden and Russia. This war, also called the Russo-Swedish War, was mainly fought in Southern Savo and it broke out due to an incident that occurred in Puumala. The battles were fought at Porrassalmi right next to Mikkeli, at Parkuinmäki in Rantasalmi and at Laitaatsilta, only a few kilometres from downtown Savonlinna. The war ended in 1790 without a clear victor.

The colourful military history of Olavinlinna

There were numerous border skirmishes between Sweden and Novgorod in the 1470–80s. In 1475, Erik Axelsson Tott started construction on a castle on an island in the Kyrönsalmi Strait that connects Lakes Haukivesi and Pihlajavesi, as this was an important transport junction. The castle was christened Olavinlinna (“Olavi’s Castle”).

The unrest in the area caused problems when building the castle, but full-scale war did not break out until 1494. Although siege was laid to Olavinlinna, the advances were beaten back.
A peace treaty was signed in 1497. The border skirmishes continued, however, and several wars were fought during the 16th century.
At the treaty of Teusina in 1595, the border was drawn so that Olavinlinna was located far away from the border in Swedish territory.

Olavinlinna was once again besieged in the Great Northern War at the beginning of the 18th century. Russian troops successfully captured it in the summer of 1714. In their opinion, capturing this strong, albeit it somewhat obsolete castle, was a major achievement and a medal was struck to commemorate this event. When signing the peace treaty in 1721, however, the Russians handed control of the castle back to Sweden.

In the Russo-Swedish War of 1741–43, Sweden lost Olavinlinna for good. The last time it was laid siege to was in 1742, when the castle surrendered after only two days. In 1788, during the next Russo-Swedish War, the Swedes tried to reconquer the castle and failed. After this, no battles have been fought over who owns Olavinlinna.

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Southern Savo suffered not only in the wars against foreign countries but also from internal conflicts already in the 16th century. In December 1596, Savo peasants joined the Club or Cudgel War, an uprising of peasants that started in Ostrobothnia. Battles were fought in Rantasalmi, Juva and in downtown Suur-Savo (part of Mikkeli now). Reinforcements joined the troops at Olavinlinna and defeated the rebelling peasants, whose villages were consequently sacked.

The Russo-Swedish War of 1808–9 was not fought in Southern Savo. As a result of the war, Finland became an autonomous Grand Duchy under Russian sovereignty and the national border dividing the region vanished.