His greatest success came during Operation Barbarossa, the 1941 invasion of the Soviet Union. During the first two months of "Barbarossa", 1st Panzer Army plunged east into Soviet territory, then moved south to the Black Sea to cut off Soviet forces in the Battle of Uman, then north to encircle Soviet forces around Kiev, then south again across the Dnieper River, and then further south to cut off Soviet forces near the Sea of Azov. Through all this strenuous campaigning, Zeitzler kept 1st Panzer Army moving smoothly and ensured that supplies arrived. In appreciation of Zeitzler, Kleist commented "The biggest problem in throwing about armies in this way was that of maintaining supplies."