In May 1941 Panzer Group 1 was attached to Field Marshal Gerd von Rundstedt's Army Group South at the beginning of Operation Barbarossa. At the start of Operation Barbarossa in June 1941, Panzer Group 1 included the III, XIV and XLVIII Army Corps (motorized) with five panzer divisions and four motorized divisions (two of them SS) equipped with 799 tanks. Panzer Group 1 served on the southern sector of the Eastern Front against the Red Army and was involved the Battle of Brody which involved as many as 3,000 Red Army tanks. The units of the Group closed the encirclement around the Soviet armies near Uman and near Kiev. After the fall of Kiev Panzer Group 1 was enlarged to become the 1st Panzer Army (on October 6, 1941) with Kleist still in command. The army captured Rostov, but was forced to retreat eight days later.