Franconicus,

There are lot of facts that show SU preparations for assault (concentration of Soviet forces on border, maps of Europe for Soviet oficiers, huge volume of supply materials on boarders etc.) and some historians speak even about data for attack (something like Soviet "Barbarossa") - 6th of July 1941. Soviets were waiting only for German invasion in Great Britain. In this case they could take over all Europe without any serious struggle, because main German forces would be involved in battle in UK.