The T-28 medium tank was deployed in the Invasion of Poland and later during the Winter War against Finland. In the course of these operations it was found that the armour was inadequate and programs were initiated to upgrade it. According to Russian historian M. Kolomietz's book "T-28. Three-headed Stalin's Monster", over 200 T-28s were knocked out during the Winter War. Frontal plates were upgraded from 50 mm to 80 mm and side and rear plates to 40 mm thickness. With this up-armoured version, the Red Army broke through the main Finnish defensive fortification, the vaunted Mannerheim Line. The Soviets thus began to upgrade their T-28 tanks for the coming war with Germany, but many were still lost during the first two months of the invasion, when the Germans invaded in June 1941.