Note, these posts are a copy of what I wrote on a similar question here.

There are several reasons for this, political and military ones, for both sides. I will start with the Finns.

Finland

Military factors

The Finnish army, while lacking in equipment, was by ww2 standards a very well-trained force. Finnish conscripts served 365 days of conscription with a for the time modern training regime. The Finnish tactics revolved around late ww1 German tactics as learned by the Finnish officers and soldiers in the German Jäger units. Defence in depth, flexible counter-attacks, foreposts at the front and most men kept safe further back during enemy barrages and tactical flexibility on the attack, with infiltration and tactical flanking were German tactics developed to deal with the ww1 battlefield. As it turned out, these were superb tactics for the close-range forest fighting and large stretches of woodlands lacking fronts completely.

The Finns realised the potential of the mortar, especially in rough terrain (as much of the fighting would be in) early, and while lack of funds prevented them from having as many as they would have wanted, they had attached them to the infantry battalions with their own forward observers (sometimes with radios, but most often telephone lines) which gave the Finnish infantry battalion some direct punch, even if the mortars did not fire within line of sight.

The Finns raised their infantry from counties and companies from muncipalities. This meant that most men knew each other from before the war, and kept unit cohesion high. There's a tendency in battle to forget politics and nationalism and fight only for your comrades. Having men you have known for a long time next to you in the trench helps immensly. High casualties could and did cause some villages to have their male population nearly wiped out by this though.

Inter-war Finnish training had, like in most Nordic countries, put some emphasis on rifle accuracy. What many contenintal nations considered "harrasing fire" with rifles, the Nordic nations trained to be sharpshooting. The Finns were especially good at this. Combined with a long and strong hunting tradition, this created a large pool of very good shots, culminating in Simo Häya who in 90 days had 542 confirmed kills.

The Finnish infantry battalion were one of the few to have integrated the SMG in the rifle squad. A Finnish platoon contained two SMG squads and two LMG squads.

Finnish infantry battalion 1939:

• 24xSMG

• 24xLMG

• 12xHMG

The Finnish air force lacked a lot of equipment, and had very few fighters. In one sense, this was a blessing in disguise as the air force, lacking in planes could be extremely selective in who it accepted into service. Only the very best of the very best among the conscripts were chosen to become fighter pilots and the training was extremely hard. Finnish pilots were required to pass extremely hard shooting tests. To add to this core of well-trained pilots, the Finnish air force was the very first adapter of the rotten-schwarm/fighting pair-finger four tactic - in 1932. The Germans developed the same tactics when fighting in Spain 1938, and the British switched after learning them the hard way over France 1940 and used them to good effect during the Battle of Britain.

Generally, Finland wasa decently egalitarian society. Many of the officers and NCOs were conscripts themselves and led by example and worked to earn the respect of the men rather than use strict discipline. Being used to long distances to central authority, these men were not beyond taking initiative without orders when the situation required it, which further increased the flexibility of the Finnish forces.

The Finnish troops were drawn from men used to long and hard winters and moving on skis during several months of the year. The Finnish army held regular winter exercises and skis were among the standard equipment of the army - in winter condition a unit on skis that have trained on how to move together can move very quickly in columns, where one man makes a track for the rest for a while, then stops, rests and lets the next man take over the hard task and then joins the last part of the column.

The Finnish army had inherited a lot of arms from the Imperial Russian army, as had the Red Army, and both sides used the same caliber on small arms, except for pistols, which meant that Finnish soldiers, always lacking ammunition, could loot the enemy dead an dload their rounds directly into their own weapons.

Political factors

While Finland has suffered a bloody civil war only 20 years earlier, the country had healed the most glaring wounds, and the threat of a foreign invasion did weld communists, socialists and whites together. Most Finnish communists who had fled to the Soviet Union had been killed in Stalin's purges, and one could not find that much support for Stalin even from the Finnish communists in Finland even before the war. During the inter-war years Finland had become a stable democracy - right wing movements such as the Lappo movement had disgraced themselves and lost all support and social reforms such as an 8 hour workday, vacation and social insurance had been enacted.

Generally, the Finnish people proved very willing to sacrifice for their nation and their democracy, and left and right united against the Soviets, something which the Soviets had not expected.

Stalin pulled out what few Finnish communists were left, several of them alcoholics, some of them from Gulag camps and created the Terijoki government (from the village where they were seated, one of the few slices of Finnish terrain the Soviets captured) under Otto Ville Kuusinen. This government proclaimed the "Democratic Republic of Finland" and signed to all Soviet demands. Leaflets were printed and distributed over Finland, often dropped by air. The promises were often completely out of touch with Finnish politics - one thing that was promised was an 8 hour workday! The Soviets expected a quick victory and to be welcomed by the Finnish communists. They were sorely dissapointed.

The first month or so of the war, the world cared little about Finland and its woes, but when it became clear that Finland was not only holding, but actually defeating Soviet invasion forces, a kind of mass hysteria of sympathy swept over the world to provide aid for Finland. While Germany, in order to keep relations with their non-aggression partner the Soviet Union, refused transit of arms and volunteers, there were still a lot of Hungarian and Italian arms that made their way to Finland. Britain and France, who both really needed to focus on re-arming themselves sent massive aid to Finland. The US sold or gave away much of its surplus arms, although most of the US weapons arrived after the war had ended.

However, nowhere did the mass hysteria reach the levels they did in Sweden. Sweden more or less depleted her stores of arms and ammunitions to send to the Finns. For example 147 000 mortar shells and 12,2 million rifle cartridges were sent to Finland.

What did they seek to gain out of it?

Finland had been part of the Russian Empire (as a sort of pseudo-autonomous Grand Duchy, whose Grand Duke just happened to be the Tsar) until 1917 and, when Russia descended into revolution and civil war, so did Finland, only there the reds lost.

(Fun fact, this is why Finland's flag is blue and white, when throughout most of her history the Finnish national colours were yellow and red)

Stalin spent some time in the early part of the war, having secured an alliance of sorts with Germany, trying to recover the borders of the old Tsarist empire. As well as occupying the parts of Eastern Poland that the USSR had lost in the Polish-Soviet war of 1919-20, he also forced the Baltic states to accept "treaties of mutual assistance" that essentially amounted to military occupation.

Finland was essentially supposed to be more of the same. The USSR asked for some "minor" territorial concessions (Petsamo, Viipuri and Hanko) and used them as a pretext for war, assuming that their large armies would have little difficulty crushing the Finns and Finland would be added back to the new Russian Empire.

Why did nobody foresee the terrain being an issue and how could a super power have been so ill prepared to invade?

Stalin had led massive purges of all parts of the administration of the USSR from 1935-39, aimed at weeding out anyone politically "suspect", and had in particular gutted the higher echelons of the Red Army, removing three of the five marshals, 13 of 15 army commanders, 8 of 9 admirals, 50 of 57 corps commanders, 154 out of 186 divisional commanders and 25 of 28 Army Corps commissars.

Although most of the men thus removed were only expelled from the party, not shot as was believed for some time, eventually returned to the service, and only represented a smallish percentage of all officers, a climate was created: fear of even seeming to criticise or disagree with Stalin.

The Soviet invasion of Poland had gone well with under 1,000 casualties, and Stalin and military strategist Voroshilov were confident that Finland would be equally easy. Some of the generals on the ground warned of the difficulties of terrain but were told to get on with it. Fearful for their jobs and lives, they did as they were told. Moreover, the dual-command system, with military decisions having to be ratified by political commissars on their political merit, also discouraged independence of command.

Finally, the Soviet leadership were mesmerised by the success of Germany's Blitzkrieg and were determined to try this themselves in Finland. The armies were thus grouped to carry out this kind of operation, entirely oblivious of the fact that the Finnish lakes and forests were nothing like the Polish steppe; that Blitzkrieg required an independence of action at junior command level which they had just finished stamping out; and that their air arm, the "flying artillery", was nothing like as effective as the Luftwaffe.

There was a lot of tension between the two countries, due to what the Finns called the Heimosodat or Kinship Wars. These were wars fought between 1918 and 1922 where areas surrounding or historically linked to Finland fought to gain independence from Russia. Volunteer soldiers from Finland itself aided this movement, and many of them were inspired by the idea of Greater Finland. Greater Finland was a nationalist idea which emphasized territorial expansion of Finland. Have a map (The current borders of modern-day Finland and lands lost to the USSR in 1940-1944 are light blue, the rest are lands that Finland could have expanded into in the context of Greater Finland.)

Now, to the buildup to the war. As Nazi Germany expands, Soviet military planners begin to realise that any attack on the USSR would be likely to be many-pronged, and one of these prongs could be Finland. They were also concerned with the potential vulnerability of Leningrad. Thus, the Soviets attempted a rapprochement with Finland, asking for a secret military alliance, or in lieu of that, a written promise that Finland that the country would resist any German approach. They also wanted to install air and sea defenses on the island of Suursaari to guard the approach to Leningrad and Kronstadt. In return, the USSR would guarantee the integrity of Finnish borders.

The reasons that the Finns did not accept this treaty is more complex. The Finns generally distrusted the Soviets, although the Anschluss in Austria had caused a stir. However, many Finns continued to regard Germany as the only possible ally in case of trouble with Russia. A Soviet envoy had also mentioned to an earlier prime minister, Kivimaki, that, "In the case of war, the USSR could not avoid occupying Finnish territory." The Finns accordingly responded that any treaties with the Russians would undermine Finnish sovereignty and would run counter to Finnish policy of neutrality and Scandinavian orientation.

This then leads to a long series of complex negotiation, over which the status of the Aaland islands was a key issue, although the primary intention of the Soviets was to secure the position of Leningrad. Stalin at one point took a military map, and drew, in the presence of a Finnish envoy, where the border should run, leading one of them to note ominously that "[Stalin] was obviously well oriented in the geography of the area". Neither side is willing to give much ground, and this leads to rising hostility. The November 3rd Pravda stated bluntly that "The Soviet Union does not only have the right, but the duty, to take measures which will guarantee the security of the sea and land approaches to Leningrad". Molotov then claimed that the Finns had fired shots upon Soviet soldiers, despite the fact the shots came from the Soviet side of the border (according to a Finnish investigation at least). This then lead to war, where Soviet troops crossed the frontier in several places.

The reasons the Soviets were so ill-prepared was largely due to the purges of the Soviet army. These had crippled the "bourgeois" officer class, and left the Soviets woefully short of the middle stage of the chain of command. For example, despite the fact that the Finns had few anti-tank weapons and insufficient training in modern anti-tank tactics, the favoured Soviet armoured tactic was a simple frontal charge, the weaknesses of which could be exploited easily. Had they had officers, the weakness of this strategy would probably have been realised much more quickly.

The problem of the Red Army with regards to terrain was that it was too modern. There were virtually no roads along the front, and while the Finnish soldiers could almost all ski, hardly any Soviets could. The winter was also especially cold, and while most of the units on both sides had adequate winter gear, some Soviet units certainly didn't, and many died from frostbite at the battle of Suomussalmi. Furthermore, the Red Army lacked proper winter tents, and men had to sleep in improvised shelters. Some Soviet units had frostbite casualties as high as 10% even before crossing the Finnish border. Before the Winter War, no army had fought in such freezing conditions. In Soviet field hospitals, operations were done and limbs were amputated at −20 °C (−4 °F) while just past the canvas tent wall the temperature was −30°C.

I often hear that Finland is placed among the Axis in WWII. Were they really allied with Nazi Germany or were they just defending themselves from the Soviet Union?

NOTE: This is a copy of another post I made a while back, outlining the general path that Finland took between 1918-Cold War. Some of the other explanations are much more detailed in certain areas, but if you like you can use this as an overview.

Finally my time to shine! This has been my go-to topic in University when writing WW2/Soviet Union focused history papers, so I have read a large amount about it.

So up until the Revolution, Finland was a semi-autonomous part of the Russian Empire. It was also one of the most westernized parts of the Empire. Following the Revolution in Russia proper, Finland also fights civil war, except in this case the Whites end up winning. The Whites won in part due to the assistance provided to them by Imperial Germany. With the civil war won, Finland decides to elect itself a king, and chooses a German prince to become king of Finland. However, before their new king can be coronated, the Germans lose the war and their monarchy collapses. This leaves Finland in a spot, because they had hitched themselves to the Great Power that just got defeated. So, they go into the interwar period without any sort of Great Power guarantor. Also, as a note, the Whites were led by a general named Mannerheim, who would later lead Finland through WW2 and is considered to be the greatest Finnish statesman of all time.

Fast forward to the mid-late 1930s. Finland has good relations with the western powers, but does not have any sort of concrete military alliance. They also have good relations with other Scandinavian states, but again, they don’t have any sort of formalized alliance. Despite this, Finland’s leaders believe that the Scandinavian states will come to one another’s aid should they be invaded.

In 1934, the Soviet Union and Finland had signed a non-agression pact, and later on the Soviets would offer a formal military arrangement between the countries, should the Germans attack. This was rejected by the Finns because they did not believe it was credible, and because it was Finnish policy to attempt to keep the Soviet Union at arms length. The Soviets wanted to secure an alliance with Finland because Finland is an ideal corridor for invasion in the Soviet Union, as the Finnish border was only 32km from Leningrad. So, without an alliance, the Soviets just straight out ask for a huge piece of Finnish Karelia and islands in the Gulf of Finland, with compensatory territory being offered in the North. The Finns say no, so the Soviet Union ends up invading Finland. The invasion doesn’t go well, with heavy Soviet casualties and relatively little territory gained. The western allies waffle about what to do, at one point planning on landing in Norway and reinforcing the Finns by land. However, they do nothing, in part because they don’t want to burn bridges with the Soviets. Sweden does nothing, although there are a large number of Swedish volunteers who fight for Finland. Eventually, the two sides end fighting, with Finland ceding the territory that the Soviets had asked for as well as other concessions.

After the peace, German diplomats begin indicating off the record that Finland “everything would be made right in time.” Germany and Finland begin having much deeper cooperation, and the Finnish army begins to obtain large quantities of modern German equipment. Finland would then eventually commit to being a part of Barbarossa (called the Continuation War in Finland), and was quickly able to recapture the territory they had ceded. Rather than stopping there, the Finns continued the war and attempted to capture areas that had been culturally Finnish, losing their claim that they were fighting a defensive war. It was at this point that many of the allies declared war on Finland.

Fast forward to 1944. The war is looking bad for Germany and Finland sues for peace with the Soviets. The terms are incredibly harsh, with Finland losing the territory they had just recaptured, as well as the Petsamo nickel mining region. They were also forced to fight their former German allies in the Lapland War, although in many cases the Finnish and German militaries would avoid major confrontation.

Unlike the other Soviet conquests in the East, a communist government is not installed in Finland, although the Finnish communist party was legalized. Finland kept its democratic and market based economy, but throughout the Cold War they had to play a balancing act to keep from raising the ire of the Soviets. To sum up Finland in WW2, their situation can be traced back to not having a great power guarantor at the beginning of WW2, combined with Soviet desire to secure its western borders.

If you want sources I can certainly provide them.

I'm curious if the Soviet defeat in Finland led Germany to believe it would be easy to conquer Russia?

in years 1939 - 1941 stalin and russia were allies of hitler. they invaded Poland almost the same time hitler did (hitler: 1st September 1939, stalin: 16th Spetember 1939) splitting the country between them.

just a friendly reminder.

What level of technology did the Finland army had ??? Was it like the polands ,??( who were still fighting the German tanks with horses ).