The brunt of the Russian attach fell at first upon the frontier defences of the Finns in the Karelian Isthmus. These comprised a fortified zone about twenty miles in depht running north and south through forest country, deep in snow. This was called the "Mannerheim Line", after the Finnish Commander-in-Chief and savior of Finland from Bolshevik subjugation in 1917. The indignation excited in Britain, France, and even more vehemently in the United States, at the unprovoked attack by the enormous Soviet Power upon a small, spirited and highly-civilised nation was soon followed by astonishment and relief. The early weeks of fighting brought no success to the Soviet forces, which in the first instance were drawn almost entirely from the Leningrad garrison. The Finnish Army, whose total fighting strength was about only 200,000 men, gave a good account of themselves. The Russian tanks were encountered with audacity and a new type of hand-grenade, soon nicknamed "the Molotov Cocktail".
Churchill, Winston S. The Second Wolrd War: The Gathering Storm. 1948. Page 485.
More about the Molotov Cocktail here.