The goal of Army Group South was to drive through and seize the Ukraine, seize Kiev, and then drive to and seize the Caucasus. They began their assault on June 22, 1941. Their advance was a bit slower on this front than others, probably because of the terrain in this area, which was quite swampy and thereby difficult for tanks to advance through quickly. On August 5, Army Group South reached Odessa, which is about 275 miles south of Kiev. They then began to besiege this city, which held out for 73 days until surrendering on October 17. While this was happening, reinforcements from Army Group Center in the form of Guderian's Panzer divisions were arriving, and this allowed the Germans to begin to drive for Kiev. On August 25 they crossed the Dnieper River, which was about 230 miles northeast of Odessa. On September 15, the Germans began to attack Kiev, and the Russians surrendered on the 27th, 12 days later. This cost the Russians the keystone of the Ukraine and 665,000 men in its defense, either killed or captured. This greatly weakened the Soviet defense of the rest of the front in 1941, while reinforcements were not yet drafted and mobilized from the civilian population. After this victory, Guderian's Panzer divisions were transferred back to Army Group Center, so they could take part on the now ready to resume drive on Moscow. The rest of Army Group South marched on, taking Kharkov on October 24, capturing another Russian army and moving about another 250 miles in a month. The Germans then marched on in deteriorating weather to Rostov, which was about 250 miles southeast of Kharkov, and seized it from the Russians on November 21. However, during the Russians winter offensive, they were able to re-take it, and Army Group South retreated to behind the Mius River.