The three independent Baltic countries – Estonia, Latvia, and Lithuania – were invaded and occupied in June 1940 by the Soviet Union, under the Molotov–Ribbentrop Pact between Nazi Germany and the Soviet Union in August 1939, immediately before the outbreak of World War II. The three countries were annexed by the Soviet Union as "constituent republics" in August 1940. Most Western countries did not recognise this annexation, and considered it illegal. On 22 June 1941, Nazi Germany invaded the Soviet Union and within weeks occupied the Baltic territories. In July 1941, the Third Reich incorporated the Baltic territories into its Reichskommissariat Ostland. As a result of the Red Army Baltic Offensive of 1944, the Soviet Union recaptured most of the Baltic states and trapped the remaining German forces in the Courland Pocket until their formal surrender in May 1945.

The Baltic states or the Baltic countries is a geopolitical term encompassing Estonia, Latvia, and Lithuania. All three countries are members of NATO, the European Union, the Eurozone, and the OECD. The three sovereign states on the eastern coast of the Baltic Sea are sometimes referred to as the "Baltic nations", less often and in historical circumstances also as the "Baltic republics", the "Baltic lands", or simply the Baltics.