Russia invaded Finland at dawn on the 30th November 1939. In addition to territorial claims the Soviets sought to install their own puppet government in Finland. They portrayed their invasion as a response to a border incident on the 26th November. The British War Cabinet had already noted the likely circumstances of this:
Interest has centred on the Soviet-Finnish dispute, which flared up afresh as the result of an alleged incident on 26th November, when seven Finnish shells were said to have fallen among a party of Soviet troops; a charge which the Finnish authorities deny as they say that’s no Finnish artillery was within 20 km of the frontier. The Soviet Government alleges that the incident took place at 3:45 pm on Sunday, but they had their official protest ready to hand to the Finnish ambassador at 8:30 pm on the same night, and it seems highly probable that the incident had been foreseen. The subsequent breaking off of the diplomatic relations shows that the Soviet is determined to force the pace and at least to subject Finland to the maximum degree of intimidation which she can exert.
The prospects for Finland, a country of 3.6 million people, did not look good, she had a 1,200 km frontier with Russia to defend. Her army of 160,000 men faced an initial Russian force of 460,000 troops and over 2,000 tanks.
The Finns had long been aware of the threat that Russia posed. Most viewed it as a threat to the very existence of their country. Finland had well prepared defensive positions and her army was extremely well motivated, as well as being well adapted for fighting in the arctic conditions. The Finnish commander, Mannerheim had established a highly effective defence line across the most vulnerable part of the country, the Karelian Isthmus in the south, taking full advantage of the numerous lakes and woods to place a formidable barrier between Helsinki, the capital, and the invading forces. Further north the Russians, far from their supply lines, found themselves facing skilful, mobile forces in extremely inhospitable terrain, where they were vulnerable to ambush.
Sodatkuvina net has a click through image time line of the entire war.