On 10 July, Guderian's 2nd Panzer Group began a surprise attack over the Dnieper, his forces overran the weak 13th Army and by 13 July, Guderian had passed Mogilev, trapping several Soviet divisions. His spearhead unit, the 29th Motorised Division, was already within of Smolensk. The 3rd Panzer Group had attacked, with the 20th Panzer Division establishing a bridgehead on the eastern bank of the Dvina river, threatening Vitebsk. As both German panzer groups drove east, the 16th, 19th and 20th armies faced the prospect of encirclement around Smolensk. From 11 July, the Soviets tried a series of concerted counter-attacks. The Soviet 19th Army and 20th Army struck at Vitebsk, while the 21st and the remnants of the 3rd Army attacked against the southern flank of 2nd Panzer Group near Bobruisk.