When Operation Barbarossa, the German invasion of the Soviet Union, began on 22 June, the 172nd was stationed at the Tesnitskoye Military Camp near Tula. In late June and early July, the division was rushed to the Belarusian city of Mogilev, where it became part of the 61st Rifle Corps of the 13th Army of the Western Front. There, Romanov organized the defense of the western bank of the Dnieper, blocking the Minsk–Mogilev and Mogilev–Bobruisk roads. As well as his own unit, Romanov ultimately controlled forces from the 110th Rifle Division, regiments or remnants of the 132nd, 137th, 160th, and 143rd Rifle Divisions, in addition to the remnants of the 20th Mechanized Corps; these formed the core of the city's defense. Immediately after it arrived, the city was besieged by German troops, whose first attack was made by elements of the 3rd Panzer Group against the division on 3 July. The defenders managed to repulse the attack and successive German attempts to cross the Dnieper, but were gradually worn down after 23 days in encirclement.