World War II: The Soviet Union invades East Karelia and the previously Finnish part of Karelia, occupied by Finland since 1941.

The Soviet Union, officially the Union of Soviet Socialist Republics (USSR), was a transcontinental country that spanned much of Eurasia from its formation in 1922 until its dissolution in 1991. It was the world's third-most populous country, the largest by area, and bordered twelve other…