The northern section of Army Group South faced the Southwestern Front, which had the largest concentration of Soviet forces, and the southern section faced the Southern Front. In addition, the Pripyat Marshes and the Carpathian Mountains posed a serious challenge to the army groups northern and southern sections respectively. On 22 June, only the northern section of Army Group South attacked, but the terrain impeded their assault, giving the Soviet defenders ample time to react. The German 1st Panzer Group and 6th Army attacked and broke through the Soviet 5th Army. Starting on the night of 23 June, the Soviet 22nd and 15th Mechanized Corps attacked the flanks of the 1st Panzer Group from north and south respectively. Although intended to be concerted, Soviet tank units were sent in piecemeal due to poor coordination. The 22nd Mechanized Corp ran into the 1st Panzer Armys III Motorized Corps and was decimated, and its commander killed. The 1st Panzer Group bypassed much of the 15th Mechanized Corps, which engaged the German 6th Armys 297th Infantry Division, where it was defeated by antitank fire and Luftwaffe attacks. On 26 June, the Soviets launched another counterattack on the 1st Panzer Group from north and south simultaneously with the 9th, 19th and 8th Mechanized Corps, which altogether fielded 1649 tanks, and supported by the remnants of the 15th Mechanized Corps. The battle lasted for four days, ending in the defeat of the Soviet tank units. On 30 June Stavka ordered the remaining forces of the Southwestern Front to withdraw to the Stalin Line, where it would defend the approaches to Kiev.. What four Mechanized Corps groups fought on June 26th?