Hoepner commanded the 4th Panzer Group on the Eastern Front during Operation Barbarossa, the invasion of the Soviet Union in 1941. Units under his command closely cooperated with the Einsatzgruppen and implemented the Commissar Order that directed Wehrmacht troops to summarily execute Red Army political commissars immediately upon capture. Hoepner's Panzer group, along with the 3rd Panzer Group, spearheaded the advance on Moscow in Operation Typhoon, the failed attempt to seize the Soviet capital.