What soviet action caused concern among western nations in the period 1939-1940?

The Soviet invasion of Poland in September 1939, following the Molotov-Ribbentrop Pact with Nazi Germany, raised significant alarm among Western nations. This action not only facilitated Germany's rapid advance into Poland but also demonstrated the Soviet Union's willingness to expand its territory and influence in Eastern Europe. Additionally, the subsequent Soviet occupation of the Baltic states in 1940 further heightened fears of Soviet expansionism and the potential threat to Western democracies. These events contributed to the growing tensions that would ultimately lead to World War II.