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Frankly, I didn't think the USSR activity during that period deserved a mention, because during the period between August 1939 (Molotov-Ribbentrop pact) and June 1941 (Barbarossa) it was not a major player. How do you propose to fix it? Frankly, the situation is very complex because from 1 Sept 1939 to 22 June 1941 the USSR:
 Won a border war against Japan;
 Expanded its territory at the expense of some Central European states;
 Conducted a war against Finland;
 Negotiated about further division of the spheres of influence with Germany;
 Remained formally neutral.
How all of that can be described in few words? Probably ""... apart from formally neutral USSR, which also expanded its territory at the expense of some of its neighbours.""
From one hand, if you think it would be not too wordy for the lede, we can add that. From the other hand, I am not sure it would be absolutely correct in a situation when conquest of Norway, Danmark, Greece or Yugoslavia is left beyond the scope totally.    "