"When Nazis attacked the USSR, was there an advance warning?"
Stalin had plenty of advance warning - there were even people defecting from the German ranks and telling them that the Germans were coming (they were probably shot for their troubles).
Even after the invasion started Stalin refused to believe the reports coming from frontline units for a long time after the invasion started.
Stalin had plenty of advance warning - there were even people defecting from the German ranks and telling them that the Germans were coming (they were probably shot for their troubles).
Even after the invasion started Stalin refused to believe the reports coming from frontline units for a long time after the invasion started.
See: http://www.simonsebagmontefiore.com/stalincourtredtsar.aspx