 In the two years leading up to the invasion , the two countries signed political and economic pacts for strategic purposes . Nevertheless , on 18 December 1940 , Hitler authorized an invasion of the Soviet Union , with a planned start date of 15 May 1941 . The actual invasion began on 22 June 1941 . Over the course of the operation , about four million soldiers of the Axis powers invaded the Soviet Union along a 2 @,@ 900 @-@ kilometer ( 1 @,@ 800 mi ) front , the largest invasion force in the history of warfare . In addition to troops , the Germans employed some 600 @,@ 000 motor vehicles and between 600 @,@ 000 and 700 @,@ 000 horses . It marked the beginning of the rapid escalation of the war , both geographically and in the formation of the Allied coalition . 
