The 'Battle of Salla' was fought between Finnish and Soviet forces near Salla on the eastern side of northern Finland during the 'Talvisota' winter war (30 November 1939/13 March 1940).

In their invasion of Finland, the Soviet forces on the western side of the White Sea had been ordered to advance through Salla to Kemijärvi and Sodankylä, and from there to Rovaniemi in a period of just just two weeks. From there they were to advance to Tornio and cut Finland into northern and southern parts. The Finnish troops managed to stop the Soviet advance just to the east of Kemijärvi, and during the last days of February 1940 were replaced by the Swedish, Norwegian and Danish volunteers of the Stridsgruppen SFK.

By 1938 the USSR had decided to take Finland, which had divided from the USSR as an independent state in 1917. Relying in part on the information provided by Finnish communists, the Soviet military leadership had secured detailed intelligence on Finnish infrastructure by the summer of 1939 in a 200-page book distributed to the leaders of the invasion force. In the central part of the proposed new front, Komkor Mikhail P. Dukhavov’s 9^th Army was tasked with invading Finland between Kuhmo and Salla and cutting the country in half by advancing to the north coast of the Gulf of Bothnia. As part of the 9^th Army's offensive, the 122^nd Division, which arrived from Poland on 8 November 1939, was to take Salla and Kemijärvi and advance to Rovaniemi within two weeks, from where it would continue to Tornio near the Finnish border with Sweden. The Soviets foresaw only light resistance, and their troops were ordered not to cross the Swedish frontier.

The Soviets began building a railway from Kandalaksha, on the western shore of the White Sea, to the Finnish border in 1939 using 100,000 prisoners as slave labour. During the late 1930s, moreover, existing roads were improved and new roads were constructed westward from the railway linking Murmansk and Leningrad to the Finnish border: one of these was the road from Kandalaksha to Alakurtti.

These and other improvements in the Soviet infrastructure and demographics near the border made it possible for the Soviets to base and supply some 40,000 troops in the region, but this had little effect on Finnish operational planning in northern Finland. The Finnish general staff did not believe that the USSR would launch a major offensive from the White Sea region into Finland. As a result, work on fortifying key road chokepoints in northern Finland began only in the autumn of 1939.

The Finnish forces in northern Finland were under the command of the staff of the Lapin Ryhmä (Lapland group), which was subordinate to the Pohjois-Suomeen Ryhmä (North Finland group). The Finns had one detached battalion (Er.P 17) and one company (Er.K Kojonen) near Salla, and these were to conduct an active defence of the region by crossing the border, stopping the advance of the single Soviet regiment that was expected in the area, and harassing the Soviet lines of communications, thereby tying down the Soviet forces. The Finnish general staff came to consider the force insufficient for even this task, but could spare no more troops from the defence of the more important Karelian isthmus. As Finland undertook a general mobilisation in October 1939, the troops had time to take stock of the situation and came to the conclusion that even limited offensive operations across the border were beyond their capabilities, and thus training was subsequently limited to defensive and delaying operations.

A Finnish intelligence estimate on 15 October placed one Soviet division in the area between Murmansk and Kandalaksha, but expected that there would be a larger Soviet force concentration in the future. On 30 November, the Soviets in fact had four divisions in the area.

Four-fifths of southern Lapland are covered by forest or swamp. The geography is dominated by forest-covered fells that surround large swamps and lakes. In December 1939 the surface s of the lakes and swamps were not yet sufficiently frozen to support motor vehicle movement but this became irrelevant as the winter progressed and the temperature dropped to -40° C (-40° F), which also made military operations more difficult.

Movement of roads was thus impossible for larger military formations. There was only one road from the Soviet border to Salla. The road network was better developed to the west of Salla, with numerous small roads to support lateral movements and the encirclement of defending forces.

There were only small numbers of Finnish troops in the area at the start of the war. The Er.P 17 (17^th Separate Battalion, also known as the 'Salla' Pataljoona), was mobilised before the war, and its main elements came from a company of the Frontier Guard. From 11 December 1939 onward the Finnish troops were part of the Lapin Ryhmä, which was commanded by Kenraalimajuri Kurt Martti Wallenius, and from 5 December the Finns defences were to be reinforced with infantry, artillery, mortar and anti-tank units. The total number of Finnish troops in the area was some 3,500 men.

The Soviets attacked with the 122^nd Division, whose slow progress forced the Soviets to reinforce this formation late in December 1939 with the 88^th Division. Before this, the 122^nd Division had pushed past Alakurtti, been slowed by a Finnish roadblock, and reached Salla on 9 December. Here the Soviet division split into two for its continued advance. The northern part moved to the north-west along the road to Savukoski, slowed by one roadblock, and then to the south-west through another Finnish roadblock to reach the Kemijoki river at Pelkosenniemi on 16 December. The Soviet division’s southern part moved to the south-west along the road through Märkäjärvi to Kemijärvi on the Kemijoki river, which it neared on 17 December after fighting its way through one roadblock before being halted by another.

Soviet troops had thus pushed the Finns back to the Kemijoki river, but had been unable to break through the Finnish defences along the river. The Soviet supply lines were now some 90 miles (145 km) long, ahd the Finns took advantage of the overstretched Soviet position by launching attacks with ski troops on the Soviet lines of communication, whose protection demanded the use of about one-third of the available Soviet troops, and on 13 January, the 9^th Army ordered the southern part of the 122^nd Division to retreat to the village of Märkäjärvi, which it did on 14 January, pursued by the Os. Roininen.

Four days later, on 18 December, the northern part of the 122^nd Division was outflanked to the north by the Os. Suoranta, and then driven back from Pelkosenniemi and back toward Salls, but managed to halt on 23 December about three-fifths of the way from Savukoski to Salla.

For the next two months the battle was a stalemate with the exception of small skirmishes and exchanges of artillery fire. On 13 March, the last day of the war, the Soviets launched a major fire preparation with artillery, aircraft and infantry weapons as part of a planned renewal of offensive operations toward Rovaniemi. The Scandinavian volunteers, who had replaced the Finns from 10 February, suffered their most casualty-intensive day of the war, with 10 killed and 30 wounded.

Overall, Finnish casualties were 1,100 men, including 650 killed or missing. Scandinavian volunteer casualties were 33 dead, 50 wounded and 130 frostbitten. Soviet losses have been estimated as 4,000 men.