The Battle of Koljonvirta (Finnish: Koljonvirran taistelu, Swedish: Slaget vid Virta bro) i.e. the Battle of the Virta Bridge, also known as the battle of Idensalmi, was fought between Swedish and Russian troops on October 27, 1808. The Swedish force consisted of troops from Savolax and Östergötland. After the main Swedish army had been defeated at the Battle of Oravais the army under Johan August Sandels in Savonia had to retreat in order not to be outflanked by the Russians. Sandels found a good defensive position north of Idensalmi (now Iisalmi) and decided to resist the Russian advance there.

Between September 29 and October 27 a cease-fire was in effect. Sandels was heavily outnumbered but had a good defensive position between two lakes connected by the Koljonvirta river and he had prepared his position well during the cease-fire. On October 27 the cease-fire was to end at 1 PM, but Russians started their attack a little earlier, perhaps because of the time difference between Sweden and Russia. Sandels pulled back the forces on the south side of the river and the Russians attacked over the partially demolished bridge. The Swedes counter-attacked and literally pushed the Russian troops into the river. The Russians pulled up fresh troops on the south side of the river, but did not try to attack again. The battle was the last Swedish victory on Finnish soil.

The Swedes had between 1,200 and 1,800 men present with 12 guns; or 4,000 men according to Russian accounts. The Russians attacked with between 5,000 and 6,000 men; taking in consideration the Russian muster roll of 1 November, above 5,800 men were present at the battle if its losses are applied. According to Swedish sources, the Russians had between 7,000 and 8,000 men with 17 guns. The Swedes suffered 34 killed and 282 wounded, compared to 221 killed, 479 wounded and 73 captured Russians, or 764 killed and missing. Swedish sources estimate their losses to over 1,000 NCOs and privates, and 32 officers killed or wounded, as well as 72 NCOs and privates and two officers captured.

• Sven Dufva

• Hornborg, Eirik (1955). När riket sprängdes: fälttågen i Finland och Västerbotten, 1808-1809 (in Swedish). Stockholm: P. A. Norstedts och Söners Förlag.
• Generalstaben, Krigshistoriska avdelningen (1910). Sveriges krig åren 1808 och 1809, Volume 5 (in Swedish). Stockholm: Kongl. boktryckeriet P. A. Norstedt & söner.
• "Иденсальми". Military Encyclopedia: In 18 Volumes. 1911–1915.

63°35′19″N 27°09′51″E / 63.58861°N 27.16417°E / 63.58861; 27.16417