At the start of 1940, Stalin knew that his army had defeated the Japanese at Khalkhin Gol (Nomonhan) in 1939. Once he has finished the annoying war with Finland, it must have been very tempting to use the free year before either the Anglo-French alliance or (less likely) Germany emerges battered but victorious from their war to decisively defeat Japan so that he can fight the surviving capitalists for the control of Eastern Europe without distractions. Obviously, he cannot trust the capitalist powers not to turn on him, so he must prepare his attack in secret and launch it only once he is sure that his European potential enemies are locked in combat. As his intelligence is quite good, he probably suspects that the Germans are planning an attack in May. Thus the Soviet Army in the Far East prepares for a May assault, receives the command to attack as soon as ready on May 11th and launches its attack on May 13th.
How would the attack go? How would Japan's relations with Britain and the USA have been affected (note that Britain was unwilling to supply goods to Japan in 1939-40 because they could be sent via the USSR to Germany and that US public support for intervention fell when Hitler attacked Russia)? Would the Battle of Britain have seemed a good idea to Germany?