Soviet foreign policy during the interwar years can best be summed up as 'isolationist'. This means that the USSR was not interested in involving itself very closely in the affairs of other countries. In this respect its policies were similar to those of the USA in the same period. The Soviet government was above all concerned to be left alone. It largely abandoned its earlier hope that other countries might experience communist revolutions of their own. It was anxious not to do anything that might provoke capitalist governments to intervene as they had done after 1917. In the course of the 1920s, diplomatic relations were established with some of the USSR's former enemies and a limited amount of trade between the USSR and these countries began to develop. Its closest ties were with Germany, a power that was also isolated from the other European states. The Treaty of Rapallo (1922) established trade links between Germany and the USSR and allowed the Germans to evade the terms of the Versailles treaty by training soldiers on Russian soil.

When Hitler came to power in Germany in 1933, Stalin at first hoped that some kind of Russo-German understanding could continue. It became more and more apparent, however, that Hitler meant what he said about making Germany once again into a great power. Stalin therefore began to turn to other countries as a way of checking Hitler's ambitions. In 1934 the USSR was admitted to the League of Nations. In the following year mutual defence treaties were signed with France and Czechoslovakia. From 1936 Stalin sent support to the Republican forces in the Spanish Civil War, whose enemies, the Nationalists, were receiving help from Germany. The Comintern called in vain for an international alliance against Fascism. In 1936 Germany and Japan responded by signing an Anti-Comintern Pact, joined by Italy in 1937 and Spain in 1939. German troops reoccupied the demilitarized Rhineland but Britain and France merely looked on disapprovingly, and did not respond. It seemed to the Soviet Union that the capitalist states were hoping that fascism might eliminate communism for them. These fears increased considerably after the 1939 when Britain and France refused to stop Hitler occupying Czechoslovakia, which was close to the USSR.

By 1939 Stalin had two choices:

  • Try to reach an agreement with the West to resist Germany, a difficult task considering the West's suspicious and hostile attitude towards the Soviet Union.

  • Come to an agreement with Hitler.

He chose the latter, and on 23 August 1939 the Nazi-Soviet Non-Aggression Pact was signed. Secret clauses divided Poland between the two powers and accepted the future Soviet annexation of Finland and the Baltic States.