Winter War
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. The Soviet 44th and parts of the 163rd Rifle Division, comprising about 14,000 troops, were almost completely destroyed by a Finnish ambush as they marched along the forest road. A small unit blocked the Soviet advance while Finnish Colonel Hjalmar Siilasvuo and his 9th Division cut off the retreat route, split the enemy force into smaller mottis, and then proceeded to destroy the remnants in detail as they retreated. The Soviets suffered 7,000–9,000 casualties; the Finnish units, 400. The Finnish troops captured dozens of tanks, artillery pieces, anti-tank guns, hundreds of trucks, almost 2,000 horses, thousands of rifles, and much-needed ammunition and medical supplies. So sure of their victory had the Soviets been that a military band, complete with instruments, banners and notes, was traveling with the 44th Division to perform in a victory parade. The Finns found their instruments among the captured materiel.

Battles in Finnish Lapland: The Finnish area of Lapland, bestriding the Arctic Circle, is sparsely developed, with little daylight and persistent snow-cover during winter; the Finns expected nothing more than raiding parties and reconnaissance patrols. Instead, the Soviets sent full divisions. On 11 December, the Finns rearranged the defence of Lapland and detached the Lapland Group from the North Finland Group. The group was placed under the command of Kurt Wallenius.
In southern Lapland, near the village of Salla, the Soviet 88th and 122nd Divisions, totaling 35,000 men, advanced. In the Battle of Salla, the Soviets proceeded easily to Salla, where the road split. Further ahead was Kemijärvi, while the fork to Pelkosenniemi led northwest. On 17 December, the Soviet northern group, comprising an infantry regiment, a battalion, and a company of tanks, was outflanked by a Finnish battalion. The 122nd retreated, abandoning much of its heavy equipment and vehicles. Following this success, the Finns shuttled reinforcements to the defensive line in front of Kemijärvi. The Soviets hammered the defensive line without success. The Finns counter-attacked, and the Soviets retreated to a new defensive line where they stayed for the rest of the war.
To the north was Finland's only ice-free port in the Arctic, Petsamo. The Finns lacked the manpower to defend it fully, as the main front was distant at the Karelian Isthmus. In the battle of Petsamo, the Soviet 104th Division attacked the Finnish 104th Independent Cover Company. The Finns abandoned Petsamo and concentrated on delaying actions. The area was treeless, windy, and relatively low, offering little defensible terrain. The almost constant darkness and extreme temperatures of the Lapland winter benefited the Finns, who executed guerrilla attacks against Soviet supply lines and patrols. As a result, the Soviet movements were halted by the efforts of one-fifth as many Finns.

Aerial warfare: 

Soviet Air Force: The USSR enjoyed air superiority throughout the war. The Soviet Air Force, supporting the Red Army's invasion with about 2,500 aircraft (the most common type being Tupolev SB), was not as effective as the Soviets might have hoped. The material damage by the bomb raids was slight as Finland offered few valuable targets for strategic bombing. For example, the city of Tampere was one of the most important targets because it was an important railway junction, and also housed State Aircraft Factory and the Tampere Linen and Iron Industry premises, which manufactured munitions and weapons, including grenade launchers. Often, targets were village depots with little value. The country had few modern highways in the interior, therefore making the railways the main targets for bombers. Rail tracks were cut thousands of times but the Finns hastily repaired them and service resumed within a matter of hours. The Soviet Air Force learned from its early mistakes, and by late February instituted more effective tactics.
The largest bombing raid against the capital of Finland, Helsinki, occurred on the first day of the war. The capital was bombed only a few times thereafter. All in all, Soviet bombings cost Finland five per cent of its total man-hour production. Nevertheless, Soviet air attacks affected thousands of civilians, killing 957. The Soviets recorded 2,075 bombing attacks in 516 localities. The city of Viipuri, a major Soviet objective close to the Karelian Isthmus front, was almost levelled by nearly 12,000 bombs. No attacks on civilian targets were mentioned in Soviet radio or newspaper reports. In January 1940, the Soviet Pravda newspaper continued to lie that no civilian targets in Finland had been struck, even accidentally. It is estimated that the Soviet air force lost about 400 aircraft because of inclement weather, lack of fuel and tools, and during transport to the front. The Soviet Air Force flew approximately 44,000 sorties during the war.

Finnish Air Force: At the beginning of the war, Finland had a small air force, with only 114 combat planes fit for duty. Missions were limited, and fighter aircraft were mainly used to repel Soviet bombers. Strategic bombings doubled as opportunities for military reconnaissance. Old-fashioned and few in number, aircraft offered little support for Finnish ground troops. In spite of losses, the number of planes in the Finnish Air Force rose by over 50 per cent by the end of the war. The Finns received shipments of British, French, Italian, Swedish and American aircraft.
Finnish fighter pilots often flew their motley collection of planes into Soviet formations that outnumbered them 10 or even 20 times. Finnish fighters shot down 200 Soviet aircraft, while losing 62 of their own on all causes. Finnish anti-aircraft guns downed more than 300 enemy aircraft. Often, a Finnish forward air base consisted of a frozen lake, a windsock, a telephone set and some tents. Air-raid warnings were given by Finnish women organised by the Lotta Svärd. The top scoring fighter ace was Jorma Sarvanto, with 12.83 victories. He would increase his tally during the Continuation War.

Naval warfare: 

Naval activity: There was little naval activity during the Winter War. The Baltic Sea began to freeze over by the end of December, impeding the movement of warships; by mid-winter, only ice breakers and submarines could still move. The other reason for low naval activity was the nature of Soviet Navy forces in the area. The Baltic Fleet was a coastal defence force which did not have the training, logistical structure, or landing craft to undertake large-scale operations. The Baltic Fleet possessed two battleships, one heavy cruiser, almost 20 destroyers, 50 motor torpedo boats, 52 submarines, and other miscellaneous vessels. The Soviets used naval bases in Paldiski, Tallinn and Liepāja for their operations.
The Finnish Navy was a coastal defence force with two coastal defence ships, five submarines, four gunboats, seven motor torpedo boats, one minelayer and six minesweepers and at least 5 icebreakers. The two coastal defence ships, Ilmarinen and Väinämöinen, were moved to harbour in Turku where they were used to bolster the air defence. Their anti-aircraft guns shot down one or two planes over the city, and the ships remained there for the rest of the war. At 18 January, Finnish armed icebreaker Tarmo was severely damaged at Kotka, received 2 bombs from a Soviet bomber with 39 Finnish troops killed in action. As well as coastal defence, the Finnish Navy protected the Ålandish and Finnish merchant vessels in the Baltic Sea.
Soviet aircraft bombed Finnish vessels and harbours and dropped mines into Finnish seaways. Still, only five merchant ships were lost to Soviet action. World War II, which had started before the Winter War, proved more costly for the Finnish merchant vessels, with 26 lost due to hostile action in 1939 and 1940.

Coastal artillery: Finnish coastal artillery batteries defended important harbours and naval bases. Most batteries were left over from the Imperial Russian period, with 152 mm (6.0 in) guns being the most numerous. Finland attempted to modernise its old guns and installed a number of new batteries, the largest of which featured a 305 mm (12.0 in) gun battery on the island of Kuivasaari in front of Helsinki, originally intended to block the Gulf of Finland to Soviet ships with the help of batteries on the Estonian side.