By that time, a situation no less favourable for the Soviets had emerged on the Voronezh Front’s left wing. In the offensive zone of this front’s formations, the Germans lacked the strength for a reliable closure of the gap that had had come into being in this sector of the front. The absence of any large German forces on the area of the Oskol and Seversky Donets rivers offered considerable prospects for the conduct of an offensive toward Kharkov.

The concept of ​​the 'Voronezh-Kastorno Offensive Operation' was based on the notion of encircling and then destroying the bulk of the German 2^nd Army with a blow along the axes converging on Kastorno, on freeing the railway linking Yelets and Valuyki, and thus creating the conditions for an offensive against Kursk and Kharkov. The main blow toward Kastorno was to be delivered by General Major Nikolai P. Pukhov’s 13^th Army of the Bryansk Front, as well as General Leytenant Kirill S. Moskalenko’s 40^th Army and the IV Tank Corps of the Voronezh Front. In the latter’s sector, two auxiliary attacks were also planned for General Leytenant Nikandr E. Chibisov’s 38^th Army and General Major Ivan D. Chernyakovsky’s 60^th Army. Air support for the undertaking was the task of General Major Ivan G. Piatykhin’s 15^th Air Army of the Bryansk Front and part of General Major Konstantin N. Smirnov’s 2^nd Air Army of the Voronezh Front.

In total, the two fronts fielded 27 infantry divisions, seven infantry brigades, the entire artillery of the Soviet supreme command reserve, two tank corps, eight separate tank brigades, three separate tank regiments and two separate tank battalions, and it was planned that 72% of the infantry, 90% of the artillery and all of the tank forces would be concentrated on the primary attack axes.

The start of the offensive elements set by the two front commanders was 24 January for the 40^th Army, 25 January for the 60^th Army, and 26 January for the 38^th Army and 13^th Army. Except for the 38^th Army, which was to attack in a single echelon, the armies were instructed to attack in two echelons.

The artillery preparation for both fronts involved 3,664 pieces of artillery and mortars, and the average artillery densities per kilometre of front in the breakthrough sectors were 92 for the 13^th army, 50 for the 38^th Army and 60^th Army, and 40 for he 40^th Army. The planning for the use of artillery on both fronts was carried out at army level, and the duration of the artillery preparation was to be 65 minutes for the 13^th Army, 90 minutes for the 38^th Army, 100 minutes for the 60^th Army and 30 minutes for the 40^th Army. Continued artillery support for the attack was planned in for only 13^th Army.

To create artillery support groups for the infantry, each division was reinforced with an average of two or three artillery or mortar regiments.

Army artillery groups were formed in the 13^th Army, 40^th Army and 60^th Army, and included two regiments in the 40^th Army and one division for the 13^th Army. The 13^th Army also created an army rocket artillery group consisting of one artillery division, two rocket artillery regiments and one separate rocket artillery battalion.

Air support for the 15^th Army was 407 serviceable aircraft, and about 120 aircraft were drawn from the 2^nd Air Army. The plan for the combat use of the 15^th Air Army envisaged an air preparation for the attack in the 13^th Army’s breakthrough sector.

In the 13^th Army, operating on a 28.5-mile (46-km) front, the strike group was concentrated on an 11.25-mile (18-km) front between the Kshen and Olym rivers. In the strike group’s first echelon there were four infantry divisions, one tank brigade and three tank regiments, and in its second echelon three infantry divisions and the 129^th Tank Brigade. The front’s reserve of two infantry divisions and one tank corps was concentrated in the 13^th Army’s sector.

In the 38^th Army, the strike group was deployed on an 8.75-mile (14-km) front from Kozinka to Ozerka and had two infantry divisions, one tank brigade and one tank battalion, with all of these forces in a single echelon. Behind the shock group in army reserve were one infantry division, a school for junior lieutenants and one tank battalion.

The 60^th Army took over from the 40^th Army a 13.67-miles (22-km) sector of the front between the Don river and Semidesyatsk, and deployed its strike group on the left flank, on a 7.5-mile (12-km) sector, a first echelon with two infantry divisions, one infantry brigade and three tank brigades; in the second echelon was one infantry division.

The 40^th Army operated on a front of 31 miles (50 km) and in its first echelon deployed five infantry divisions, one infantry brigade and one tank brigade; the second echelon comprised three ski and infantry brigades and the 305^th Division, which was moving out of the Alekseyevka area into the army’s offensive zone. The IV Tank Corps took up its initial positions in the area of the 309^th Division and 107^th Division, and had been allotted the immediate task, together with these divisions, of breaking through the Axis forces' defences and then developing any initial success toward Kastorno. The main forces of the 40^th Army were concentrated in the centre on an 18.5-mile (30-km) front with four infantry divisions and all its armoured strength in the first echelon.

The first stage of the operation began on 24 January on the southern flank as the 40^th Army and IV Tank Corps attacked. The 40^th Army’s offensive developed only slowly. A blinding blizzard reduced the effectiveness of the artillery barrage, and the attackers had to overcome deep snow drifts. Because of the snow’s depth, the fighting was fought mainly along the roads. The Germans, lacking prepared defensive lines, nevertheless offered stubborn resistance in settlements, all of which which had been turned into centres of resistance. By the morning of the next day, therefore, the 40^th Army’s infantry divisions had been able to advance only some 3.1 to 3.7 miles (5 to 6 km). The IV Tank Corps also failed to fulfil its role as it was unable to break away from the infantry units as they all moved along a single road, which was passable only by medium tanks. Therefore, all of the light tanks, as well as wheeled vehicles, lagged behind. Even so, on 25 January the German resistance was broken.

On the morning of 25 January, the front commander demanded that the 40^th Army and IV Tank Corps increase the pace of their offensive. The infantry and armour soon broke the enemy’s resistance, and by the end of the day had advanced to a depth of between 12.5 and 15.5 miles (20 to 25 km). The rapidity of the tank and infantry formations' advance threaten to encircle the Axis forces in the area of Voronezh, and the latter began a hasty withdrawal. On the same day Voronezh was liberated.

On 25 January, the 60^th Army launched its offensive from the east, and completely cleared Voronezh of German troops.

On 26 January, the offensive on the Soviet forces' northern flank began as the 13^th Army and 38^th Army moved forward. By this time the weather had improved, and this made it possible for the 15^th Air Army to provide a useful level of support. By the end of the day, the German defences had been broken.

By the evening of 26 January, a corridor about 37.25 miles (60 km) wide still remained behind the German troops to the west of Voronezh, but the Soviets saw the opportunity for their armoured forces to seal this possible escape route. On the following day, the 13^th Army, 38^th Army and 60^th Army went onto the offensive. The 13^th Army and 38^th Army broke through the German defences, and by the end of 27 January had advanced between 6.1 and 12.5 miles (10 and km) in various sectors. The 60^th Army’s efforts to advance were less success, its divisions managing to penetrate the German defences to a depth only only between 1.85 and 3.1 miles ((3 and 5 km). At the same time, the 40^th Army liberated Gorshechnoye, an important centre of German resistance, and captured the garrison of Stary Oskol in an attack on three sides. On the same day, General Major Andrei G. Kravchenko’s IV Tank Corps, reinforcing the 40^th Army, struck out to the north in a deep penetration. As the supply of fuel was hampered by drifting snow, the delivery of fuel for immediate needs was partially undertaken by Polikarpov U-2 light biplane aircraft. By the evening of 27 January, the IV Tank Corps had managed to reach the southern outskirts of Kastorno, which was the rendezvous fixed for the linking of the convergent attack axes. On the following day, the advance detachments of the 13^th Army and 38^th Arm approached the village from the north and east respectively, and the Soviet assault on the village began. On 29 January, Kastorno was liberated, and thus the communications of eight German and two Hungarian divisions of General Ernst-Eberhard Hell’s VII Corps and General Erich Straube’s XIII Corps were cut.

On 28 January, shock groups of the 13^th Army, 40^th Army and 38^th Army began a rapid advance toward Kastorno, and this was soon cleared of all Axis forces. Thus the encirclement of the Axis forces in the area of Voronezh and Kastorno had been completed.

However, the Soviet forces had not yet been able to establish a continuous front around the Axis group, and no measures had yet been undertaken to destroy the tapped forces. Between Kastornoye and Bykovo, that is the area between the IV Tank Corps in the area of Kastorno and Lachinovo, and the 40^th Army’s 25^th Guards Division in the area of Bykovo, there was a gap of 15.5 miles (25 km) not occupied by Soviet troops. A gap of the same size also remained between Gorshechnoye and Stary Oskol. On the entire 31-mile (50-km) front between Kastorno and Stary Oskol, there were only overstretched units of the 25^th Guards Division. In this situation, the Soviet supreme command ordered Golikov and Reyter to leave only the smallest forces possible to complete the destruction of the encircled Axis forces, and thus to concentrate their efforts in the development of success toward Kursk and Kharkov with their main forces.

Golikov set new tasks for his forces: the 38^th Army was to strike at Oboyan, the 40^th Army at Belgorod, and the 60^th Army at Kursk. Thus the destruction of the trapped Axis forces was relegated to a secondary significance, for it was believed that this task would be completed in a short time by limited forces and without detriment to the pace of the main offensive. Thus only part of the 38^th Army was involved and, as a result, no continuous Soviet front could be created round the pocket.

The encirclement of a large Axis force between Kastorno and Voronezh opened the prospect of liberating a large area of Soviet territory, and the Soviet supreme command continued to believe that it was of primary importance for the two fronts to advance deeply before the Axis forces were able to stabilise their front and thus replace that which had just been lost. For this reason, the Soviet forces involved in the operation deployed to the west to attack Stary Oskol, Kursk and Kharkov, leaving the destruction of the Axis pocket to the 38^th Army. After it had achieved this, the 38^th Army was to join the attack on Kharkov.