The 21st Mechanized Corps was a formation in the Soviet Red Army during the Second World War. Initially formed in March 1941, in response to the German victories in the West it was attached to the newly forming 27th Army, and held in reserve near Opochka in Soviet Union 130 kilometres (81 mi) South of Pskov in the Special Baltic Military District. It was under the command of Major General Lelyushenko when the German Operation Barbarossa began in June 1941. It initially consisted of the 42nd and 46th Tank Divisions, and the 185th Motorized Division. a After the Invasion began the Special Baltic Military District was renamed Northwestern Front, Commanded by Colonel General Kutznetsov. The front fielded 8th and 11th Armies with the 27th Armies in its second echelon. The 21st Mechanized was heavily engaged in the first battles of Operation Barbarossa, particularly during the Baltic Operation (1941). After the spectacular advances by 4th Panzer Group Kutznetsov asked Stavka for the release of Berzarin's 27th Army and the 21st Mechanized Corps on 25 June and was ordered to halt Erich von Manstein's LVI Panzer Corps which was closing up to the Daugava River later that day both units engaged near Dvinsk.

The 185th Rifle Division was formed as an infantry division of the Red Army just as the Second World War was beginning, in the Oryol Military District, based on the pre-September 13, 1939 shtat. As a standard rifle division it took part in the Soviet occupation of Lithuania in June 1940. In early 1941 it was selected for conversion to a motorized infantry division, but in fact it had received little in the way of vehicles by the start of the German invasion and so was a rifle division in all but name. It was sent to join Northwestern Front in the Baltic states with its 21st Mechanized Corps, and was soon assigned to 27th Army. Under these commands the division retreated through July and August as the German 16th Army advanced behind in the general direction of Novgorod. On August 25 it was officially reorganized as a regular rifle division.