When reading about the Russo-Finnish Winter War I found this very interesting. In addition, to the surprise of both the Soviet leadership and the Finns, it turned out that the majority of the Finnish Socialists did not support the Soviet invasion but fought alongside their compatriots against the common enemy. Many Finnish Communists had moved to the Soviet Union in the 1930s to "build Socialism," only to end up as victims of Stalin's Great Purges, which led to widespread disillusion and even open hatred of the Soviet regime among Socialists in Finland. This partial healing of the wounds and rifts after the Civil War in Finland (1918), and Finland's language strife, is still referred to as "the Spirit of the Winter War," although it should also be noted that many communists were not allowed to fight in Finland's conscription army because of their political background. Soviet arrogance and/or incompetence was an important factor. The attackers weren't expecting much of a struggle, and due to Stalin's purges, the commanders of the Red Army had suffered 80% peacetime losses. These were commonly replaced by people less competent but more pleasing to their superiors, and tactics which were obsolete by World War I were sometimes witnessed. Tactics were strictly "by the book," as a failed initiative otherways carried a high risk of execution. The Soviet army was also far less well prepared for winter warfare, particularly in forests, and heavily used vulnerable motorized vehicles. The so-called "Raatteentie Incident," during the month-long Battle of Suomussalmi, where one Soviet division was defeated after marching on a forest road straight into an ambush with vastly outnumbered Finnish soldiers, is still used in military academies as an example of what not to do. http://www.absoluteastronomy.com/encyclopedia/w/wi/winter_war.htm --
I don´t know how many top foreign communists were asked to visit Moscow by Stalin or the Party in the 1930´s but anyway several of them never returned, I know many Finns never came back and also several Polish ones. the Finnish communists knew what would happen to them because " a true communist did not live outside USSR" in Stalin´s view, they were spies or the sort. Some kinda reason to send them to Gulag anyway. So they decided if not take part in Winter War at least not interfere by causing sabotage.
You have to wonder at how gullible some Communists were. That they wouldn't notice what had happen to others and Stalin's leanings towards killing people with no real reason other then in his head.
If I recall correctly Tito was the only leader ever to have refused to pack his bags and go to Moscow once the invitation was sent....that is post-WW2 of course!
You must remember that the Finnish Communist Party (SKP) was illegal back in 1939. Its organization comprised underground cells led from Moscow by Finns controlled by the Soviet CP. EK (Etsivä Keskuspoliisi, Finnish civilian counter-espionage service or police) had quite efficiently infiltrated the SKP network in Finland. But the fact that influenced the SKP supporters to remain loyal with the Government was the Ribbentrop pact. For them it was incomprehensible how Stalin could make friends with Nazi Germany.
Also the infamous Otto-Wille Kuusinen government, that Stalin had created immediately after the Winter War had started. If Stalin said he only wanted the islands and some land from karelian isthmus to cover Leningrad, this new government meant that he wanted the whole country. As well there was a symphony made for the conquer of Helsinki...
Despite of everything, the USSR has received the full control over water area of Ladoga lake and has secured Murmansk which was near to the Finnish territory (peninsula Fishing). Soviet Union has received experience of conducting war during winter time, in wood and marshy territory, experience of break of long-term strengthenings and struggle against the opponent applying tactics of guerrilla war.