Finnish Karelia in the context of Säyneinen

The Finnish Democratic Republic (Finnish: Suomen kansanvaltainen tasavalta or Suomen kansantasavalta, Swedish: Demokratiska Republiken Finland, Russian: Финляндская Демократическая Республика), also known as the Terijoki Government (Finnish: Terijoen hallitus), was a short-lived puppet government of the Soviet Union in occupied Finnish territory from December 1939 to March 1940.

The Finnish Democratic Republic was established by Joseph Stalin upon the outbreak of the Winter War and headed by Otto Wille Kuusinen to govern Finland after Soviet conquest. The Finnish Democratic Republic was only recognised by the Soviet Union and nominally operated in Soviet-occupied areas of Finnish Karelia from the de facto capital of Terijoki. The Finnish Democratic Republic was portrayed by the Soviet Union as the official socialist government of Finland capable of restoring peace, but lost favor as the Soviets sought rapprochement with the Finnish Government. The Finnish Democratic Republic was dissolved and merged into the Karelo-Finnish SSR upon the signing of the Moscow Peace Treaty.

The Karelian Isthmus (Russian: Карельский перешеек, romanized: Karelsky peresheyek; Finnish: Karjalankannas; Swedish: Karelska näset) is the approximately 45–110-kilometre-wide (30–70 mi) stretch of land situated between the Gulf of Finland and Lake Ladoga in northwestern Russia, to the north of the River Neva. Its northwestern boundary is a line from the Bay of Vyborg to the westernmost point of Lake Ladoga, Pekonlahti. If the Karelian Isthmus is defined as the entire territory of present-day Saint Petersburg and Leningrad Oblast to the north of the Neva and also a tiny part of the Republic of Karelia, the area of the isthmus is about 15,000 km (5,800 sq mi).

The smaller part of the isthmus to the southeast of the old Russia-Finland border is considered historically as Northern Ingria, rather than part of the Karelian Isthmus itself. The rest of the isthmus was historically a part of Finnish Karelia. This was conquered by the Russian Empire during the Great Northern War in 1712 and included within the autonomous Grand Duchy of Finland (1809–1917) of the Russian Empire. When Finland became independent in 1917, the isthmus (except for the territory roughly corresponding to present-day Vsevolozhsky District and some districts of Saint Petersburg) remained Finnish. Finnish Karelia was partly ceded to the Soviet Union by Finland following the Winter War (1939–1940) and Continuation War (1941–1944). In 1940–1941, during the Interim Peace, most of the ceded territories in the isthmus were included within the Karelo-Finnish SSR. However, since World War II the entire isthmus has been divided between the city of Saint Petersburg (mostly Kurortny District), as well as Priozersky District, Vsevolozhsky District and Vyborgsky District of Leningrad Oblast.

Ostrobothnia (Swedish: Österbotten; Finnish: Pohjanmaa) is a historical province comprising a large portion of western and northern Finland. Before the Treaty of Fredrikshamn in 1809, Ostrobothnia was part of Sweden. It is bounded by Karelia, Savo, Tavastia (Häme) and Satakunta in the south, the Bothnian Sea, Bothnian Bay and Swedish Norrbotten in the west, Lapland in the north and Russia in the east.

Savo, or Savonia (Finnish: Savo; Swedish: Savolax), is a historical province in the east of Finland. It borders Tavastia, Ostrobothnia and Karelia. Savo is now divided into two provinces: North Savo and South Savo. The largest cities in Savo by population are Kuopio, Mikkeli, Savonlinna, Varkaus and Iisalmi.