It was then involved in the Soviet invasion of Poland (1939). It entered the Second World War as part of the Soviet Southwestern Front, comprising the
13th Rifle Corps (including the 44th, 58th, and 192nd Rifle Divisions)
17th Rifle Corps (60th Mountain, 69th Mountain, and 164th Rifle Divisions),
16th Mechanised Corps (15th, 39th Tank Divisions, 240th Mechanised Division)
10th, 11th and 12th Fortified Regions, and artillery, engineering and other units.
It participated in the frontier battle to the west of Stanislau. In the second half of July as part of the Soviet Southern Front it conducted defensive fights in the direction of Uman. During the Battle of Uman, the Twelfth Army was caught in a huge encirclement south of Kiev along with the 6th Army. Thus the army's headquarters was disbanded on 10 August 1941, after the battle.
The 12th Army was reformed in August 1941 as part of the Soviet Southern Front on the basis of 17th Rifle Corps. On 1 September 1941 its structure included 270th and 274th Rifle Divisions, the 11th Tank Division, 268th and 374th Corps Artillery Regiments, 64th and 181st Fighter Aviation Regiments, and a number of separate formations.
The Army defended the left coast of the Dnepr around Zaporozhye, from the end of September to the beginning of December, 1941. It participated in the Donbass defensive, Rostov defensive and offensive, in January, 1942 in the Barvenkovo–Lozovaya Offensive operations, in the subsequent conducted defensive fights in Donbass and on Northern Caucasus (part of the Battle of the Caucasus). In the middle of April 1942 the 261st Rifle Division under Colonel A.M. Ilina, the 4th Rifle Division (Colonel I.P.Roslogo), the 74th Rifle Division under General F.E.Sheverdina, the 176th Rifle Division (General V.N.Martsinkevicha) and 54th Tank Brigade (тбр) under Colonel K.S. Minarova were assigned to the Army.
It was later in 1942 reorganised as a defensive zone HQ, but then reformed again by conversion of the previous 5th Tank Army in mid April 1943. It joined the Southwestern Front. Its structure included the 172nd, 203rd, 244th, 333rd and 350th Rifle Divisions and other formations. In April - July the Army was in Front reserve, and then participated in the Donbass and Zaporozhye offensive operations. In November the army HQ was disbanded, with its forces transferred to other armies.
Active 1939 to 1945
Major General Ivan Tyulenev, September 1939 to October 1939
Colonel Filipp Parusinov, October 1939 to 10 March 1941
Major General Pavel Ponedelin, 11 March 1941 to 10 August 1941
Major General Ivan Galanin, 25 August 1941 to 16 October 1941
Major General Konstantin Koroteyev, 17 October 1941 to 5 April, 1942
Major General Andrei Grechko, 5 April 1941 to 3 September 1942
Colonel Aleksandr Sokolski, 4 September 1942 to 13 September 1942
Lieutenant General Nikolai Kirichenko, 13 September to 20 September 1942
Lieutenant-General Ivan Shlemin, 20 April 1943 to 19 May 1943
Major-General Alexei Danilov, 20 May 1943 to 30 October 1943
13th Rifle Corps - Major General Nikolai Kirillov
44th Mountain Division - Mj.Gen. S.A. Tkachenko
58th Mountain Division - Mj.Gen. N.I. Proshkin
192nd Mountain Division - Col. S.D. Gubin
17th Rifle Corps - Major General Ivan Galanin
60th Mountain Division - Mj.Gen. M.B. Salikhov
96th Mountain Division - Mj.Gen. I.M. Shepetov
164th Rifle Division - Col. A.N. Chervinskii
16th Mechanized Corps - Major General A.D. Sokolov
15th Tank Division - Col. V.I. Polozkov
39th Tank Division - Col. N.V. Starkov
240th Motor Rifle Division - Col. I.V. Gorbenko