During World War II, in the course of Operation Barbarossa, Nazi Germany invaded Estonia in July–December 1941, and occupied the country until 1944. Estonia had gained independence in 1918 from the then warring German and Russian Empires. However, in the wake of the August 1939 Nazi-Soviet Pact, the Stalinist Soviet Union had invaded and occupied Estonia in June 1940, and the country was formally annexed into the USSR in August 1940.