The Russo-Swedish War of 1554–1557, a precursor to the larger Livonian War, had significant consequences for Finland, then part of the Kingdom of Sweden. Tensions between Sweden and Russia escalated into conflict after a series of border skirmishes and the raid of the Pechenga Monastery by Swedish forces. Russia responded with a large-scale invasion of Finnish territory.
In 1555, Russian forces numbering up to 20,000 soldiers invaded Finland, overwhelming the small initial Finnish defense force. Sweden quickly sent reinforcements, including infantry and cavalry, many of whom were Finnish nobles. Swedish-Finnish forces attempted to seize Russian fortresses like Oreshek, but the sieges were unsuccessful due to poor planning and lack of supplies.
The war saw significant fighting around the key town of Viborg (Vyborg), which Russia threatened in 1556. Although the Russians pillaged the surrounding areas, they unexpectedly withdrew, leaving Viborg unconquered. This spared Finland from further devastation.
The conflict ended with the Treaty of Novgorod in 1557, which preserved the pre-war borders and granted merchants free passage between Sweden and Russia. For Finland, the war underscored its strategic importance in the ongoing power struggles between Sweden and Russia, as its territory frequently became a battleground in their conflicts.