 The Germans had begun massing troops near the Soviet border even before the campaign in the Balkans had finished . By the third week of February 1941 , 680 @,@ 000 German soldiers were gathered in assembly areas on the Romanian @-@ Soviet border . In preparation for the attack , Hitler moved more than 3 @.@ 2 million German and about 500 @,@ 000 Axis soldiers to the Soviet border , launched many aerial surveillance missions over Soviet territory , and stockpiled war materiel in the East . 
