Putin cites many (dubious) historical precedents for invading Ukraine. While Russia flounders in their second year of attempting to seize Ukrainian farmland, the Fins should reclaim what is theirs.
There are parts of Russia that were formerly Finnish territory. After the Finnish Declaration of Independence in 1917, the border between Finland and Russia was established by the Treaty of Tartu in 1920. However, following the Winter War (1939-1940) and the Continuation War (1941-1944) between Finland and the Soviet Union, Finland was forced to cede significant territories to the Soviet Union under the Moscow Peace Treaty (1940) and the Treaty of Paris (1947). These territories include the Karelian Isthmus, parts of Salla and Kuusamo, and the Petsamo area. Today, these regions are part of the Russian Federation.