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.

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    Welcome to History:SE. Aren't the reasons covered in the Wikipedia article? Perhaps you could edit your question to explain why you think the reasons given there aren't a sufficient answer? – sempaiscuba Apr 5 '19 at 13:38
  • Much better. +1 – sempaiscuba Apr 6 '19 at 0:39
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    WRT "...the Finnish troops were exhausted and short of ammunition...", you may know this now, but did the Soviets know it then? You may have little doubt that the Soviets could have conquered Finland, but 1) that's your opinion; and 2) based on hindsight. It's the opinion of the Soviet leadership that mattered. – jamesqf Apr 6 '19 at 3:41
  • Just a remark: the Baltic states had been annexed in July-August 1940, months after the end of the Winter war. In 1939 they remained independent, but had to accept the Soviet military bases. – jmster Apr 6 '19 at 18:20
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    Khrustchev memoirs provides an answer of sorts. – Moishe Kohan Apr 8 '19 at 21:02

The Soviets were indeed winning, but there were several reasons to stop:

  • They'd basically fulfilled the demands they made of Finland before starting the war. Those were mainly about moving the border so that Leningrad wasn't so close to Finland.

  • Making an aggressive war against a much smaller country was becoming politically embarrassing, even to Stalin.

  • The spring thaw was on the way, which presented a definite risk of the Red Army becoming bogged down in mud.

  • There was a risk that the French and/or British would intervene. This was their intention, but they'd been unable, so far, to get Norwegian and Swedish agreement to move troops through those countries. If Finland was swallowed by the USSR, that might very well change.

  • The Red Army had lost a lot of troops and equipment. While the Finnish army was running out of men and equipment, the Finnish people would have been really hard work to rule. They'd been part of the pre-revolutionary Russian Empire, and they hadn't liked it one bit.

  • The USSR was notionally allied with Germany, but weakening the Red Army a lot further for a gain that was not essential risked Germany taking advantage. Stalin never trusted anyone, especially not Hitler. While Stalin was surprised by the timing of Operation Barbarossa, he had been preparing for war with Germany for a long time. He seems to have convinced himself that the German preparations that had been detected were an attempt to trick the USSR into attacking before its build-up was complete.

Making peace just looked like a good idea, I think.

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    Do you have sources for the last two points? About the fifth point: Finland was almost at the end of their resistance, so the would not be able to continue the war much longer. About the last point: Since Stalin was surprised by the invasion of Soviet Union on 1941, I have doubts about it as well. – Santiago Apr 5 '19 at 17:54
  • Added sources and explanation on last two points. – John Dallman Apr 5 '19 at 18:48
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    The real reason is N4. If Britain interfered, then SU would remain one-on-one against Germany, with no allies. This is what Stalin (who planned to attack Germany) was afraid of most of all. – Alex Apr 6 '19 at 16:14
  • Alex, if Stalin was afraid of that, why did he attack Finland, in the first place? – Sandra Apr 8 '19 at 15:20
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    The initial attack was predicated on the idea that the victory would be easy. You might not fear weakening yourself relative to a potential future conflict with Germany if you expect a three-week war with minimal losses. You might fear weakening yourself for a potential future conflict with Germany after 300,000 casualties. – tbrookside Apr 8 '19 at 19:50

The relevant archives are still closed and, I think, will continue to be closed for a long time. One, thus, has to look for memoirs of former Soviet leaders (of that time). Three of them wrote such memoirs: Kaganovich, Mikoyan and Khrushchev. The first two say nothing of interest (regarding your question), but Khrushchev (in book 1) did have something to say on this matter.

The most relevant pages in Khrushchev's memoirs are 254-255. Here is what he writes:

Once again the question comes up: Could the Soviet-Finnish war have been avoided? I will not take it on myself to try to draw a definitive conclusion. If we are to talk about Stalin, on the other hand, the person who decided these questions, he did not begin the war in 1939 in order to seize Finland. We did not in fact try to occupy that country, and when the Finnish army was effectively defeated in 1944, Stalin demonstrated statesman like wisdom then, too. The territory of Finland with its population could not decide any fundamental questions of foreign policy for us. It was a small nationality, whose country was not very rich in natural resources; but the signing of an armistice between ourselves and Finland, which then declared war against Germany—that was a good example for other satellites of Hitler’s Germany to see. The advantage for us was greater than occupation. Besides, the step that we did take left positive traces as far as prospects for the future were concerned.

As everything written by a (former) Soviet official, this has to be taken with a (rather large) dose of salt. Here is my reading between the lines:

  1. Initially, the Soviet leadership was expecting a "quick and victorious war". Stalin was more than happy to make Finland a "people's democracy" (see item 3 below), like Mongolia in 1920s or Eastern European countries in late 1940s. This is what Kuusinen's government was formed for and there is no other good explanation for its formation.

  2. As we all know, things did not go well and by mid-March 1940 (after Red Army finally broke the Mannerheim line and took Vyborg) Stalin has decided that Finland was not worth further efforts. Khrushchev writes of strategic insignificance of Finland in the context of the 2nd world war, but the cited paragraph concludes his discussion of the Winter War of 1939-40, so, in my reading, applies to this war as well.

  3. Lastly (but this is just my take, Khrushchev said nothing about this): Further conquest of Finland in 1940 would have taken more time and troops. Even after that, USSR would need a significant presence of Red Army and NKVD on Finnish land in order to pacify it. At the same time, USSR needed the regular troops and NKVD for the takeover of Baltic republics (and Bessarabia) in Summer of 1940, which were much more strategically significant. For the Baltic republics, at least initially, the original plan for Finland also served as a blueprint (per this wikipedia article):

In May 1940, the Soviets turned to the idea of direct military intervention, but still intended to rule through puppet regimes.[55] Their model was the Finnish Democratic Republic, a puppet regime set up by the Soviets on the first day of the Winter War.[56]

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