Winter War
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. It was composed of two divisions under Juho Heiskanen, who was soon replaced by Woldemar Hägglund.
The North Finland Group was a collection of White Guards, border guards and drafted reservist units under Wiljo Tuompo.

Soviet invasion: 

Start of invasion and political operations: On 30 November 1939, Soviet forces invaded Finland with 21 divisions, totalling 450,000 men, and bombed Helsinki, killing about 100 citizens and destroying more than 50 buildings. In response to international criticism, Soviet Foreign Minister Vyacheslav Molotov stated that the Soviet Air Force was not bombing Finnish cities but rather dropping humanitarian aid to the starving Finnish population; the bombs were sarcastically dubbed Molotov bread baskets by Finns. The Finnish statesman J. K. Paasikivi commented that the Soviet attack without a declaration of war violated three separate non-aggression pacts: the Treaty of Tartu, which was signed in 1920, the non-aggression pact between Finland and the Soviet Union, which was signed in 1932 and again in 1934; and also the Covenant of the League of Nations, which the Soviet Union signed in 1934. Field Marshal C.G.E. Mannerheim was appointed Commander-in-Chief of the Finnish Defence Forces after the Soviet attack. In a further reshuffling, Aimo Cajander's caretaker cabinet was replaced by Risto Ryti and his cabinet, with Väinö Tanner as foreign minister because of opposition to Cajander's prewar politics. Finland brought the matter of the Soviet invasion before the League of Nations. The League expelled the Soviet Union on 14 December 1939 and exhorted its members to aid Finland.
Headed by Otto Wille Kuusinen, the Finnish Democratic Republic puppet government operated in the parts of Finnish Karelia occupied by the Soviets, and was also referred to as the "Terijoki Government", after the village of Terijoki, the first settlement captured by the advancing Red Army. After the war, the puppet government was reabsorbed into the Soviet Union. From the very outset of the war, working-class Finns stood behind the legitimate government in Helsinki. Finnish national unity against the Soviet invasion was later called the spirit of the Winter War.

First battles and Soviet advance to Mannerheim Line: The array of Finnish defence structures that during the war started to be called the Mannerheim Line was located on the Karelian Isthmus approximately 30 to 75 km (19 to 47 mi) from the Soviet border. The Red Army soldiers on the Isthmus numbered 250,000, facing 130,000 Finns. The Finnish command deployed a defence in depth of about 21,000 men in the area in front of the Mannerheim Line to delay and damage the Red Army before it reached the line. In combat, the most severe cause of confusion among Finnish soldiers was Soviet tanks. The Finns had few anti-tank weapons and insufficient training in modern anti-tank tactics. According to Trotter, the favoured Soviet armoured tactic was a simple frontal charge, the weaknesses of which could be exploited. The Finns learned that at close range, tanks could be dealt with in many ways; for example, logs and crowbars jammed into the bogie wheels would often immobilise a tank. Soon, Finns fielded a better ad hoc weapon, the Molotov cocktail, a glass bottle filled with flammable liquids and with a simple hand-lit fuse. Molotov cocktails were eventually mass-produced by the Finnish Alko alcoholic-beverage corporation and bundled with matches with which to light them. 80 Soviet tanks were destroyed in the border zone engagements.
By 6 December, all of the Finnish covering forces had withdrawn to the Mannerheim Line. The Red Army began its first major attack against the Line in Taipale – the area between the shore of Lake Ladoga, the Taipale river and the Suvanto waterway. Along the Suvanto sector, the Finns had a slight advantage of elevation and dry ground to dig into. The Finnish artillery had scouted the area and made fire plans in advance, anticipating a Soviet assault. The Battle of Taipale began with a forty-hour Soviet artillery preparation. After the barrage, Soviet infantry attacked across open ground but was repulsed with heavy casualties. From 6 to 12 December, the Red Army continued to try to engage using only a single division. Next, the Red Army strengthened its artillery and deployed tanks and the 150th Rifle Division forward to the Taipale front. On 14 December, the bolstered Soviet forces launched a new attack but were pushed back again. A third Soviet division entered the fight but performed poorly and panicked under shell fire. The assaults continued without success, and the Red Army suffered heavy losses. One typical Soviet attack during the battle lasted just an hour but left 1,000 dead and 27 tanks strewn on the ice. North of Lake Ladoga on the Ladoga Karelia front, the defending Finnish units relied on the terrain. Ladoga Karelia, a large forest wilderness, did not have road networks for the modern Red Army. The Soviet 8th Army had extended a new railroad line to the border, which could double the supply capability on the front. On 12 December, the advancing Soviet 139th Rifle Division, supported by the 56th Rifle Division, was defeated by a much smaller Finnish force under Paavo Talvela in Tolvajärvi, the first Finnish victory of the war.
In Central and Northern Finland, roads were few and the terrain hostile. The Finns did not expect large-scale Soviet attacks, but the Soviets sent eight divisions, heavily supported by armour and artillery. The 155th Rifle Division attacked at Lieksa, and further north the 44th attacked at Kuhmo. The 163rd Rifle Division was deployed at Suomussalmi and ordered to cut Finland in half by advancing on the Raate road. In Finnish Lapland, the Soviet 88th and 122nd Rifle Divisions attacked at Salla. The Arctic port of Petsamo was attacked by the 104th Mountain Rifle Division by sea and land, supported by naval gunfire.

Operations from December to January: 

Weather conditions: The winter of 1939–40 was exceptionally cold with the Karelian Isthmus experiencing a record low temperature of −43 °C (−45 °F) on 16 January 1940. At the beginning of the war, only those Finnish soldiers who were in active service had uniforms and weapons. The rest had to make do with their own clothing, which for many soldiers was their normal winter clothing with a semblance of insignia added. Finnish soldiers were skilled in cross-country skiing. The cold, snow, forest, and long hours of darkness were factors that the Finns could use to their advantage. The Finns dressed in layers, and the ski troopers wore a lightweight white snow cape. This snow-camouflage made the ski troopers almost invisible so that they could more easily execute guerrilla attacks against Soviet columns. At the beginning of the war, Soviet tanks were painted in standard olive drab and men dressed in regular khaki uniforms. Not until late January 1940 did the Soviets paint their equipment white and issue snowsuits to their infantry.
Most Soviet soldiers had proper winter clothes, but this was not the case with every unit. In the Battle of Suomussalmi, thousands of Soviet soldiers died of frostbite. The Soviet troops also lacked skill in skiing, so soldiers were restricted to movement by road and were forced to move in long columns. 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.