The Soviet Union invaded Finland on the 30th of November 1939 until the 13th of March 1940. The Soviets attacked with a total 998,100 men and the Finns attacked with 337,000 to 346,500 men. After Joseph Stalin's Great Purge, there was a lack of experienced officers in the Red Army, limiting his ability to capture the country. As proven time again, the Finns defending their homeland against a foreign invader by putting up a significant resistance. The Finns also had sufficient knowledge of their terrain and used tactics such as ski's to outmaneuver and flank their enemies. Despite these obstacles, the Soviets succeeded in capturing much of the country. However, it was only after they took massive casualties.
The main aims of the Russian plan were;
- to occupy Petsamo, the Finn's Artic port, to discourage intervention from the sea,
- launch columns against Rovaniemi and Oulu,
- to penetrate the zone of pill-boxes, block-houses, tank traps, wire and land mines along the Mannerheim Line.
Although the Russians succeeded in capturing Petsamo, they were repeatedly trapped and defeated by the Finns, who had been able to inflict destructive defeats on the Russian's 44th, 54th and 18th divisions.
The fighting died down after an interval, while the Russians formed their siege train, and reorganised their assault. The state of the weather had prevented Russian aircraft carrying out reconnaissance missions, and bombing missions against Finnish bases and communications, before the second week of January 1940. The second phase of the campaign then opened on 1 February with a heavy bombardment, resulting in pressure against the fortress of Taiple on Lake Ladoga, and the coastal fort of Koivista, as well as the two anchors on the Mannerheim Line. The ten miles astride the Viipuri railway also came under continuous artillery and bombing attack. The Finns began to withdraw after the capture of Summa on 15 February. 
By 19 February the Russians were only four miles from Viipuri, and had cut off Koivisto - the southern anchor of the Mannerheim Line - on their left. On 21 February Koivisto surrendered and the Russians occupied a number of islands west of Viipuri, prompting the Finns to withdraw a week later. On 1 March Viipuri was surrounded on three sides. Five days later a Finnish delegation left for Moscow. 
On 7 March the Russians crossed the frozen Gulf of Finland to a point sixty miles west of Viipuri and occupied the city of Kotka, which surrendered on 13 March. 
A peace treaty between Finland and Russia, which was signed two hours after the surrender of Kotka, led to the surrender of Viipuri with the railway to Suojarvi and the islands of the Gulf of Finland. The Hango Peninsula at the mouth of the Gulf of Finland was leased to Soviet Russia. During the summer Hango and Viipuri were fortified.