Britain and France planned to send a expeditionary force to finland, which would bring them to war with both germany and the USSR. If they did send a expeditionary force, how would the Winter War end? and also, how would the outside world (mainly japan and the US) react?
Britain’s expeditionary forces to Norway in 1940 and Greece in 1941 were humiliating failures that ended with Dunkirk-style evacuations (Dunkirk itself being the evacuation of another British expeditionary force), so I doubt that sending a Franco-British expeditionary force to Finland in 1939 would have been of much military help to the Finns against the Russians. What makes this scenario interesting, however, is that it might have scrambled the landscape of political alliances in various ways, with unpredictable consequences for the rest of the war. This was a time when alliances were still in flux – one example being Japan’s angry reaction to the signature in August 1939 of the Molotov-Ribbentrop Pact by Germany, which had co-signed with Japan the Anti-Comintern Pact of 1936. Japan had been defeated by Zhukov at Khalkhin Gol a few days before the Molotov-Ribbentrop Pact was signed, so it was furious to see Germany sign a non-aggression treaty with Russia. (Ironically, Japan did precisely the same thing in April 1941, seven months after the signing of the Tripartite Pact with Germany and Italy.)