To me, this is one of the biggest history mysteries. By the end of February 1940, the USSR breached the Mannerheim line, and the Finnish troops were exhausted and short of ammunition. I have little doubt that at that moment, the USSR could easily conquer Finland and either absorb it or make it a vassal state. Why did the USSR not do this?
UPDATE: In response to a comment, I want to add that I read many materials, including the relevant Wikipedia pages (page 1, page 2) and ''Memoirs of the Second World War'' by Churchill, but found no satisfactory answer to my question, as explained below.
Churchill gives no reasons or hypotheses why the Soviets stopped and concluded such a peace agreement, whereas Wikipedia only provides seemingly unrealistic reasons:
(a) ''For the Soviets, casualties were high ... ''
But the USSR had already breached the Mannerheim line, and the Finnish forces were rapidly approaching exhaustion. The USSR had paid a price in terms of casualties to reach a position from which it could relatively easily conquer the entire Finland, and those casualties were a sunk cost, i.e., a cost that had already been incurred and could not be recovered by concluding a peace with the Finns.
Also, I do not think that casualties were really of great concern to the Soviet government. After all, if casualties had been of great concern, the USSR would have planned and executed the campaign much better. The Soviet government saw its own people rather as renewable resources.
The losses the USSR had suffered were ~150,000 killed and about the same number of wounded, and it was not much on the global scale at all, at least not enough to substantially affect the balance of power between the USSR and any other power. The population of the USSR was about 170 million people at that time.
(b) ''... the situation was a source of political embarrassment to the Soviet regime.''
But the USSR had already attacked Finland and had already participated in the division of Poland. It is hard to see how continuation of the Winter War would have led to any additional substantial reputation losses.
I guess that if the USSR had cared that much about its reputation as a non-expansionist state, it would not have later annexed the Baltic states, just months after the Winter War, so I highly doubt that the ''political embarrassment'' was the real reason why the USSR stopped the Winter War.
Moreover, the military reputation of the USSR had been seriously damaged by the first stages of the Winter War, and the USSR could have repaired its military reputation by continuing the war and fighting the Finns in a more proper way from the military standpoint - better planning and execution, more resources, etc. The USSR was in an excellent position to repair its military reputation, as the Mannerheim line had already been breached. The military reputation damage suffered by the USSR in the Winter War may have been one of the factors that led Hitler to the decision to attack the USSR.
(c) ''... there was a risk of Franco-British intervention.''
But if the USSR had been afraid of that, the Soviets would probably not have started the war, in the first place.
France and Britain were already at war with Germany and had no obvious geopolitical interest to defend Finland. Yes, I know that the Allies had plans of intervention with a pretext of helping the Finns, but the planned operation was actually aimed at cutting off shipments of Swedish iron ore to Germany and was intended to be very limited, with the Allied offers to Finland being 20,000-50,000 volunteers only. No full-scale war against such a big and distant power as the USSR was or could be planned by the Allies. As Churchill writes only about volunteers, I guess no war would have been declared by the Allies against the USSR if the Allies had intervened, i.e., the intervention would have been a proxy war in which French and British volunteers would have somewhat helped Finland.
No ultimatum was given by the Allies to the USSR regarding the Winter War, and no diplomatic pressure by the Allies was applied to the Soviet Union to stop the war. On the contrary, to have a pretext for the planned operation, the Allies needed the Winter War going on and thus simply had no interest to try to make the USSR conclude a peace with the Finns.
(d) ''With the spring thaw approaching, the Soviet forces risked becoming bogged down in the forests.'
But the USSR would have conquered Finland anyway, and I do not think that the approaching thaw was a major obstacle. At most it could have caused a delay.
(e) The USSR allegedly did not want to conquer Finland in full and only wanted to secure the border.
But the USSR established a puppet government headed by Kuusinen, and also the war was portrayed in the USSR as a war against ''White Finns'' or ''Capitalist Finns.'' There was even a popular war song in which the Soviet troops were portrayed as liberators of Finland. It is also hard to see what principle differences between Finland and the Baltic states were there for the Soviet government.
Wikipedia does not substantiate the above hypotheses (a)-(e) by referring to any historic documents or memoirs of people who knew or heard how the decision to stop the war had been taken by the Soviet government.
UPDATE 2. I am especially interested to see whether there is any evidence, direct or indirect, indicating what the Soviet leadership actually thought and how it took the decision to stop the war.