At the same time, Hitler's rise to power in Germany, with his policy of expansionism and anti-communism, resulted in an even graver threat to Soviet security. While seeking alliances with other powers, especially France and Britain, to counter these threats, the Soviet Union joined the League of Nations in 1934. During the succeeding five years, the Soviet commissar for foreign affairs, Maksim M. Litvinov, repeatedly urged the members of the league to take concerted action against the successive aggressions of the Fascist powers. The Soviet Union sought also to obtain support for this action, which it called collective security, by encouraging the formation in foreign countries of so-called united-front, or popular-front, governments. The united-front policy called for collaboration of Communist, socialist, and centrist political groups to oppose Fascist movements.

In the summer of 1938 a grave crisis arose when the German government demanded the cession by Czechoslovakia of the Sudetenland, a border area with a large German minority. The Soviet Union announced its readiness to support the Czechs in resisting this demand and called upon France and Britain to offer similar aid. The French and British governments, instead, accepted Hitler's assurance that this demand embodied the final territorial acquisition sought by Germany. The result was the Munich Pact of September 1938, providing for the cession of the disputed areas to Germany. The signing of this pact signalled the failure of the Soviet collective-security policy. In March 1939 the Germans advanced eastwards from the Sudetenland into Czechoslovakia and quickly took full control.