After Operation Barbarossa began on 22 June, Dyakonov left the academy and in September took command of the 27th Separate Cadet Rifle Brigade. Until November the brigade fought in heavy defensive battles near Moscow, then was relocated to the Kalinin Front and fought in the destruction of the German forces around Lake Seliger, the capture of Molotitsy and the encirclement of the Demyansk Pocket. On 29 January then-Colonel Dyakonov took command of the 257th Rifle Division of the 3rd Shock Army, which he led in the Toropets–Kholm offensive. Its units advanced more than 380 km in fighting from Ostashkov to Velikiye Luki, then transitioned to the defensive. In the spring of 1942, the 31st Rifle Brigade and two ski battalions were placed under the control of the division, expanding its defensive line from 40 to 80 km. During the Battle of Velikiye Luki, the division distinguished itself in the capture of the city and other fortified points, for which it was converted into the 91st Guards Rifle Division on 18 April.