Winter War
He stated the objective as being to secure Finland from being used as a staging ground by means of regime change.
Others argue against the idea of a complete Soviet conquest. American historian William R. Trotter asserted that Stalin's objective was to secure Leningrad's flank from a possible German invasion through Finland. He stated that "the strongest argument" against a Soviet intention of full conquest is that it did not happen in either 1939 or during the Continuation War in 1944 even though Stalin "could have done so with comparative ease". Bradley Lightbody wrote that the "entire Soviet aim had been to make the Soviet border more secure". In 2002, Russian historian A. Chubaryan stated that no documents had been found in Russian archives that support a Soviet plan to annex Finland. Rather, the objective was to gain Finnish territory and to reinforce Soviet influence in the region.
Another American historian Stephen Kotkin also shares the position that the Soviet Union did not aim for annexation. He points out the different treatment Finland was given, compared to the Baltics: unlike the pacts of mutual assistance that the Baltics were pressured into, resulting in their total Sovietization, the Soviets demanded limited territorial concessions from Finland, and even offered land in return, which would not have made sense if full Sovietization was intended. And according to Kotkin, Stalin seemed to be genuinely interested in reaching an agreement during the negotiations: he had personally attended six of the seven meetings with the Finns, and had multiple times reduced his demands. However, mutual mistrust and misunderstandings would mar the negotiations, producing an impasse.

Opposing forces: 

Soviet military plan: Before the war, Soviet leadership had expected total victory within a few weeks. The Red Army had just completed the invasion of eastern Poland at a cost of fewer than 4,000 casualties after Germany attacked Poland from the west. Stalin's expectations of a quick Soviet triumph were backed up by politician Andrei Zhdanov and military strategist Kliment Voroshilov, but other generals were more reserved. Red Army Chief of Staff Boris Shaposhnikov advocated a narrow-front assault right on the Karelian isthmus. Additionally, Shaposhnikov argued for a fuller build-up, extensive fire support and logistical preparations, a rational order of battle and the deployment of the army's best units. Zhdanov's military commander, Kirill Meretskov, reported, "The terrain of coming operations is split by lakes, rivers, swamps, and is almost entirely covered by forests.... The proper use of our forces will be difficult". These doubts were not reflected in Meretskov's troop deployments, and he publicly announced that the Finnish campaign would take two weeks at most. Soviet soldiers had even been warned not to cross the border mistakenly into Sweden.
Stalin's purges in the 1930s had devastated the officer corps of the Red Army; those purged included three of its five marshals, 220 of its 264 division or higher-level commanders and 36,761 officers of all ranks. Fewer than half of all the officers remained. They were commonly replaced by soldiers who were less competent but more loyal to their superiors. Unit commanders were overseen by political commissars, whose approval was needed to approve and ratify military decisions, which they evaluated based on their political merits. The dual system further complicated the Soviet chain of command and annulled the independence of commanding officers.
After the Soviet success at the Battles of Khalkhin Gol against Japan, on the USSR's eastern border, Soviet High Command had divided into two factions. One side was represented by the Spanish Civil War veterans General Pavel Rychagov from the Soviet Air Forces; the tank expert General Dmitry Pavlov and Stalin's favourite general, Marshal Grigory Kulik, the chief of artillery. The other faction was led by Khalkhin Gol veterans General Georgy Zhukov of the Red Army and General Grigory Kravchenko of the Soviet Air Forces. Under this divided command structure, the lessons of the Soviet Union's "first real war on a massive scale using tanks, artillery, and aircraft" at Khalkin Gol went unheeded. As a result, Russian BT tanks were less successful during the Winter War, and it took the Soviet Union three months and over a million men to accomplish what Zhukov had managed at Khalkhin Gol in ten days (albeit in completely different circumstances).

Soviet order of battle: Soviet generals were impressed by the success of German Blitzkrieg tactics, but they had been tailored to conditions in Central Europe, with its dense well-mapped network of paved roads. Armies fighting there had recognised supply and communications centres, which could be easily targeted by armoured vehicle regiments. Finnish Army centres, in contrast, were deep inside the country. There were no paved roads, and even gravel or dirt roads were scarce. Most of the terrain consisted of trackless forests and swamps. The war correspondent John Langdon-Davies observed the landscape: "Every acre of its surface was created to be the despair of an attacking military force". Waging Blitzkrieg in Finland was a highly-difficult proposition, and according to Trotter, the Red Army failed to meet the level of tactical co-ordination and local initiative that would be required to execute such tactics in Finland.
Commander of the Leningrad Military District Kiril Meretskov initially ran the overall operation against the Finns. The command was passed on 9 December 1939 to the General Staff Supreme Command (later known as Stavka), directly under Kliment Voroshilov (chairman), Nikolai Kuznetsov, Stalin and Boris Shaposhnikov. On 28 December, when Stalin asked for volunteers to take over military command, Semyon Timoshenko offered himself on the condition that he be allowed to implement Shaposhnikov's initial plan of a focused attack on the Karelian Isthmus to break the Mannerhiem Line; it was accepted. In January 1940, the Leningrad Military District was reformed and renamed "North-Western Front".
The Soviet forces were organised as follows:

The 7th Army, comprising nine divisions, a tank corps and three tank brigades, was located on the Karelian Isthmus. Its objective was to quickly overrun the Finnish defenses on the Karelian Isthmus and conquer Viipuri. From there, the 7th Army was to continue towards Lappeenranta, then turn west towards Lahti, before the final push to the capital Helsinki. The force was later divided into the 7th and 13th Armies.
The 8th Army, comprising six divisions and a tank brigade, was north of Lake Ladoga. Its mission was to execute a flanking manoeuvre around the northern shore of Lake Ladoga to strike at the rear of the Mannerheim Line.
The 9th Army was positioned to strike into Central Finland through the Kainuu region. It was composed of three divisions with one more on its way. Its mission was to thrust westward to cut Finland in half.
The 14th Army, comprising three divisions, was based in Murmansk. Its objectives were to capture the Arctic port of Petsamo and then advance to the town of Rovaniemi.

Finnish order of battle: The Finnish strategy was dictated by geography. The 1,340 km (830 mi) border with the Soviet Union was mostly impassable except along a handful of unpaved roads. In prewar calculations, the Finnish Defence Command, which had established its wartime headquarters at Mikkeli, had estimated seven Soviet divisions on the Karelian Isthmus and no more than five along the whole border north of Lake Ladoga. In the estimation, the manpower ratio would have favoured the attacker by three to one. The true ratio was much higher, however, since for example, 12 Soviet divisions were deployed north of Lake Ladoga.
Finland had a large force of reservists, which was trained in regular maneuvers, some of which had experience from the recent Finnish Civil War. The soldiers were also almost universally trained in basic survival techniques, such as skiing. The Finnish Army was not able to equip all its soldiers with proper uniforms at the outbreak of war, but its reservists were equipped with warm civilian clothing. However, the sparsely-populated highly-agrarian Finland had to draft so many of its working men that the Finnish economy was massively strained because of a lack of workers. An even greater problem than lack of soldiers was the lack of materiel since foreign shipments of anti-tank weapons and aircraft were arriving only in small quantities. The ammunition situation was alarming, as stockpiles had cartridges, shells and fuel to last only 19 to 60 days. The ammunition shortage meant the Finns could seldom afford counter-battery or saturation fire. Finnish tank forces were operationally nonexistent. The ammunition situation was alleviated somewhat since Finns were largely armed with Mosin–Nagant rifles dating from the Finnish Civil War, which used the same 7.62×54mmR cartridge that was used by Soviet forces. The situation was so severe that Finnish soldiers sometimes had to maintain their ammunition supply by looting the bodies of dead Soviet soldiers.
The Finnish forces were positioned as follows:

The Army of the Isthmus was composed of six divisions under the command of Hugo Österman. The II Army Corps was positioned on its right flank and the III Army Corps, on its left flank.
The IV Army Corps was located north of Lake Ladoga.