At 07:20 Moscow time on 19 November, Soviet forces on the northern flank of the Axis forces at Stalingrad began their offensive; forces in the south began on 20 November. Although Romanian units were able to repel the first attacks, by the end of 20 November the Third and Fourth Romanian armies were in headlong retreat, as the Red Army bypassed several German infantry divisions. German mobile reserves were not strong enough to parry the Soviet mechanized spearheads, while the Sixth Army did not react quickly enough nor decisively enough to disengage German armored forces in Stalingrad and reorient them to defeat the impending threat. By late 22 November Soviet forces linked up at the town of Kalach, encircling some 290,000 men east of the Don River. Instead of attempting to break out of the encirclement, German leader Adolf Hitler decided to keep Axis forces in Stalingrad and resupply them by air. In the meantime, Soviet and German commanders began to plan their next movements.