In “Stalin’s Other War: Soviet Grand Strategy 1939-41”, Rowman & Littlefield, Maryland, 2002, Professor Albert L Weeks suggests that Stalin used the period before Barbarossa to allow the capitalist European nations to bleed each other and to build up his forces for an attack on Germany, and possibly further into Western Europe against France and Britain, which was pre-empted by Barbarossa. http://books.stonebooks.com/cgi-bin/foxweb.exe/feedback/feedback?1007840
http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1529-921X.2003.00072.x?cookieSet=1
Weeks’ argument is more subtle and less dogmatic than indicated by my summary and the linked review as it depends upon conflicting interpretations of Soviet documents, complicated to some extent by different eras in Soviet history which influenced certain interpretations, which Weeks deals with in the book.
It is not the only interpretation possible from the documents as other writers interpret them as being consistent with aggressive defence of the USSR rather than initiating an attack on Germany.
Viktor Suvorov’s Icebreaker: Who Started the Second World War, London, Hamish Hamilton, 1990 http://www.ihr.org/jhr/v16/v16n6p22_Bishop.html looks at additional factors about military production and forces to conclude that the USSR was building up massive forces which could be used only for attack into Europe.
If such interpretations are right, it follows that the Soviet entry into the Alliance was a coincidence produced by the necessity of national survival after Germany attacked and stopped the USSR attacking Germany and replacing it as the aggressor against the rest of Europe.
In that context, the Greek campaign becomes critical as the longer it delayed the German forces the more it could delay Barbarossa and the more likely it became that Stalin could launch his attack. If Churchill had deployed adequate forces, and especially adequate air forces, in Greece he might have achieved a victory which in the long run did Britain and Western Europe a lot more longstanding harm. Maybe the best thing that happened for Western Europe was losing in Greece.