Winter War
The Red Army lacked proper winter tents, and troops had to sleep in improvised shelters. Some Soviet units incurred frostbite casualties as high as ten per cent even before crossing the Finnish border. However, the cold weather did give an advantage to Soviet tanks, as they could move over frozen terrain and bodies of water, rather than being immobilised in swamps and mud. According to Krivosheev, at least 61,506 Soviet troops were sick or frostbitten during the war.

Finnish guerrilla tactics: In battles from Ladoga Karelia to the Arctic port of Petsamo, the Finns used guerrilla tactics. The Red Army was superior in numbers and material, but Finns used the advantages of speed, manoeuvre warfare and economy of force. Particularly on the Ladoga Karelia front and during the Battle of Raate Road, the Finns isolated smaller portions of numerically superior Soviet forces. With Soviet forces divided into smaller groups, the Finns dealt with them individually and attacked from all sides.
For many of the encircled Soviet troops in a pocket (called a motti in Finnish, originally meaning 1 m3 (35 cu ft) of firewood), staying alive was an ordeal comparable to combat. The men were freezing and starving and endured poor sanitary conditions. Historian William R. Trotter described these conditions as follows: "The Soviet soldier had no choice. If he refused to fight, he would be shot. If he tried to sneak through the forest, he would freeze to death. And surrender was no option for him; Soviet propaganda had told him how the Finns would torture prisoners to death." The problem however was that the Finns were mostly too weak to fully exploit their success. Some of the pockets of encircled Soviet soldiers held out for weeks and even months, binding a huge number of Finnish forces.

Battles of the Mannerheim Line: The terrain on the Karelian Isthmus did not allow guerrilla tactics, so the Finns were forced to resort to the more conventional Mannerheim Line, with its flanks protected by large bodies of water. Soviet propaganda claimed that it was as strong as or even stronger than the Maginot Line. Finnish historians, for their part, have belittled the line's strength, insisting that it was mostly conventional trenches and log-covered dugouts. The Finns had built 221 strong-points along the Karelian Isthmus, mostly in the early 1920s. Many were extended in the late 1930s. Despite these defensive preparations, even the most fortified section of the Mannerheim Line had only one reinforced-concrete bunker per kilometre. Overall, the line was weaker than similar lines in mainland Europe. According to the Finns, the real strength of the line was the "stubborn defenders with a lot of sisu" – a Finnish idiom roughly translated as "guts, fighting spirit".
On the eastern side of the Isthmus, the Red Army attempted to break through the Mannerheim Line at the battle of Taipale. On the western side, Soviet units faced the Finnish line at Summa, near the city of Viipuri, on 16 December. The Finns had built 41 reinforced-concrete bunkers in the Summa area, making the defensive line in this area stronger than anywhere else on the Karelian Isthmus. Because of a mistake in planning, the nearby Munasuo swamp had a 1-kilometre (0.62 mi)-wide gap in the line. During the First Battle of Summa, a number of Soviet tanks broke through the thin line on 19 December, but the Soviets could not benefit from the situation because of insufficient co-operation between branches of service. The Finns remained in their trenches, allowing the Soviet tanks to move freely behind the Finnish line, as the Finns had no proper anti-tank weapons. The Finns succeeded in repelling the main Soviet assault. The tanks, stranded behind enemy lines, attacked the strongpoints at random until they were eventually destroyed, 20 in all. By 22 December, the battle ended in a Finnish victory.
The Soviet advance was stopped at the Mannerheim Line. Red Army troops suffered from poor morale and a shortage of supplies, eventually refusing to participate in more suicidal frontal attacks. The Finns, led by General Harald Öhquist, decided to launch a counter-attack and encircle three Soviet divisions into a motti near Viipuri on 23 December. Öhquist's plan was bold; however it failed. The Finns lost 1,300 men, and the Soviets were later estimated to have lost a similar number.

Battles in Ladoga Karelia and North Karelia: The strength of the Red Army north of Lake Ladoga in Ladoga Karelia surprised the Finnish Headquarters. Two Finnish divisions were deployed there, the 12th Division led by Lauri Tiainen and the 13th Division led by Hannu Hannuksela. They also had a support group of three brigades, bringing their total strength to over 30,000. The Soviets deployed a division for almost every road leading west to the Finnish border. The 8th Army was led by Ivan Khabarov, who was replaced by Grigory Shtern on 13 December. The Soviets' mission was to destroy the Finnish troops in the area of Ladoga Karelia and advance into the area between Sortavala and Joensuu within 10 days. The Soviets had a 3:1 advantage in manpower and a 5:1 advantage in artillery, as well as air supremacy.
Finnish forces panicked and retreated in front of the overwhelming Red Army. The commander of the Finnish IV Army Corps Juho Heiskanen was replaced by Woldemar Hägglund on 4 December. On 7 December, in the middle of the Ladoga Karelian front, Finnish units retreated near the small stream of Kollaa. The waterway itself did not offer protection, but alongside it, there were ridges up to 10 m (33 ft) high. The ensuing battle of Kollaa lasted until the end of the war. A memorable quote, "Kollaa holds" (Finnish: Kollaa kestää) became a legendary motto among Finns. Further contributing to the legend of Kollaa was the sniper Simo Häyhä, dubbed "the White Death" by Soviets, and credited with over 500 kills. Captain Aarne Juutilainen, dubbed "the Terror of Morocco", also became a living legend in the Battle of Kollaa. To the north, the Finns retreated from Ägläjärvi to Tolvajärvi on 5 December and then repelled a Soviet offensive in the battle of Tolvajärvi on 11 December.
In the south, two Soviet divisions were united on the northern side of the Lake Ladoga coastal road. As before, these divisions were trapped as the more mobile Finnish units counterattacked from the north to flank the Soviet columns. On 19 December, the Finns temporarily ceased their assaults due to exhaustion. It was not until the period of 6–16 January 1940 that the Finns resumed their offensive, dividing Soviet divisions into smaller mottis. Contrary to Finnish expectations, the encircled Soviet divisions did not try to break through to the east but instead entrenched. They were expecting reinforcements and supplies to arrive by air. As the Finns lacked the necessary heavy artillery equipment and were short of men, they often did not directly attack the mottis they had created; instead, they worked to eliminate only the most dangerous threats. Often the motti tactic was not applied as a strategy, but as a Finnish adaptation to the behaviour of Soviet troops under fire. In spite of the cold and hunger, the Soviet troops did not surrender easily but fought bravely, often entrenching their tanks to be used as pillboxes and building timber dugouts. Some specialist Finnish soldiers were called in to attack the mottis; the most famous of them was Major Matti Aarnio, or "Motti-Matti" as he became known.
In North Karelia, Soviet forces were outmanoeuvred at Ilomantsi and Lieksa. The Finns used effective guerrilla tactics, taking special advantage of their superior skiing skills and snow-white layered clothing and executing surprise ambushes and raids. By the end of December, the Soviets decided to retreat and transfer resources to more critical fronts.

Battles in Kainuu: The Suomussalmi–Raate engagement was a double operation which would later be used by military academics as a classic example of what well-led troops and innovative tactics can do against a much larger adversary. Suomussalmi was a municipality of 4,000 with long lakes, wild forests and few roads. The Finnish command believed that the Soviets would not attack there, but the Red Army committed two divisions to the Kainuu area with orders to cross the wilderness, capture the city of Oulu and effectively cut Finland in two. There were two roads leading to Suomussalmi from the frontier: the northern Juntusranta road and the southern Raate road.
The Battle of Raate Road, which occurred during the month-long Battle of Suomussalmi, resulted in one of the largest Soviet losses in the Winter War.