The Molotov-Ribbentrop Pact of August 1939 ensured that the Nazis and Uncle Joe's Soviet forces could carve up Poland at their leisure within 6 weeks at the start of World War II without fearing having to fight a war on two fronts. The Allied Powers could only gulp as they looked on and braced themselves for what was to become the great static Phony War in the western European theatre.
Not so in the North. Hitler cast envious eyes on Denmark, Norway and Sweden and their mineral reserves of tungsten and steel, to say nothing of heavy water for atomic research. The Russians were not so blind as to not want to check any further Teutonic encroachment within the Baltic and having beaten the living doodoo out of the straitened Poles, Stalin felt that the time was right to push for Russian naval bases in the South of Finland to avoid the Russian Baltic Fleet being bottlenecked and 'requested' a 30 year lease of the southern Finnish port of Hanko. That's what the Russians have argued, anyway.
The Finns had only regained their independence in 1917 from the Russians by splitting off during the Russian Civil War and had been helped in this task by General Mannerheim and his mainly German trained officer corps. Finland could not countenance any cession of her own territory believing, probably rightly, that this was an example of Stalinist salami-slicing and they became the original mouse that roared, confusing the Communists with their blind inability to bow to the threat from a country with an army five times their size and a population many times greater.
The Karelia region, along the SE border of Finland was the site of fierce fighting. 450,000 Soviet troops threw themselves onto the Mannerheim Line, prepared by the Finnish CiC. The Finns knew the territory imtimately and savage fighting took place in temperatures of -40 (Centigrade). The 180,000 Finns chose to harass the invaders by picking off food and supply lines in widescale guerilla tactics dispersing into small units, re-orging only just prior to another attack. E.g. At Suomussalmi the Russians lost 23,000 men compared to Finnish casualties of 800. Gen. Vinogradov was to be court-martialled and shot as a result.
By the end of February the Finns had all but exhausted their matériel and the stalemate they had forced on the Soviets was paper-thin and the government was forced to seek terms.
Terms were softened on the Russian side as realisation of what had occurred dawned in Moscow. Russian tactics had been predictable and easily neutralised by the Finnish adoption of mobile open defences with long range ski patrols able to hold up cavalry columns (aka 'mötti' tactics). The war saw the introduction of the Molotov Cocktail. The Soviet forces were mostly raised from the Ukraine, which had a tradition of naval recruitment. Soviet troops were further hampered by the issue of mittens without a trigger finger. The war brought home to Stalin the utter unpreparedness of the Red Army following his purges of all the Officer class above major and the need to radically recruit talent for the officer grades over political reliability.
Hostilities ceased on March 15th 1940 with Finland losing some territory to save Stalin's face but she retained her own sovereignty and could later form a ready platform for the Siege of Leningrad during the Continuation War of 1941-44, in tandem with Army Group North's activities.
The British alignment with Soviet Russia following the launch of Barbarossa in the summer of 1941 became a cause célèbre within Conservative circles, such as Evelyn Waugh and Brendan Bracken, as it entailed relinquishing the previous F.O. pro-Finnish position.
A small what might have been should be noted:- Prior to the surrender, an Anglo-French force had been mobilised to enter the fray on the Finnish side by crossing the neutrals Norway (prior to its invasion in April, 1940) and Sweden.