 On the eastern side of the isthmus , the Red Army attempted to break through the Mannerheim Line in the battle of Taipale . On the western side , Soviet units faced the Finnish line at Summa , near the city of Viipuri , on 16 December . The Finns had built 41 reinforced concrete bunkers in the Summa area , making the defensive line in this area stronger than anywhere else on the Karelian Isthmus . However , because of a mistake in planning , the nearby Munasuo swamp had a 1 km ( 0 @.@ 62 mi ) -wide gap in the line . During the first battle of Summa , a number of Soviet tanks broke through the thin line on 19 December , but the Soviets could not benefit from the situation because of insufficient cooperation between branches of service . The Finns remained in their trenches , allowing the Soviet tanks to move freely behind the Finnish line , as the Finns had no proper anti @-@ tank weapons . However , the Finns succeeded in repelling the main Soviet assault . The tanks , stranded behind enemy lines , attacked the strongpoints at random until they were eventually destroyed , 20 in all . By 22 December , the battle ended in a Finnish victory . 
