The 'Battle of Szack' was a battle fought between Soviet and Polish forces during the former’s seizure of eastern Poland at the time of the German 'Weiss' (i) seizure of western Poland (28 September 1939).

During the early stages of 'Weiss' (i), the Poles' Korpus Ochrony Pogranicza (Border Defence Corps) was largely stripped of all its reserves and heavy armament to bolster the country’s first- and second-line field armies as all the available Polish forces were sent to the west to reinforce the units resisting the German invasion. When the Soviets invaded eastern Poland on 17 September, therefore, there were barely any Polish forces to oppose them. The garrisons of the Korpus Ochrony Pogranicza were overstretched and, after initial clashes and skirmishes for the border forts, the Polish units had to fall back.

The deputy commander of the Korpus Ochrony Pogranicza, Generał brygady Wilhelm Orlik-Rueckemann, decided to concentrate as many troops as possible under his command and join the rest of the Polish forces in the west. He thus ordered all the Korpus Ochrony Pogranicza forces in the Polesia area to withdraw, and over a period of several days managed to gather about 9,000 men from various units spread along a 185-mile (300-km) strip of the Polish/Soviet border. On 19 September he ordered all his units to march toward Kowel, where his forces were to be joined Generał brygady Franciszek Kleeberg’s Samodzielna Grupa Operacyjna Polesie (Independent Operational Group 'Polesie'). The difficult situation and the outcome of the 'Battle of Brześć' and the 'Battle of Kobryń' forced Rueckemann’s group to change its plans, however, and the Polish units changed direction and started to march through the forests toward Włodawa and Kamień Koszyrski.

The group found itself in a no-man’s-land between the Soviet and German forces, and could operate freely although the troops' morale was low. On 27 September Orlik-Rueckemann decided to engage the Soviet forces to achieve a victory and raise morale.

The Polish forces were moving in two columns when, during the early morning of 28 September, the northern column reached the forests near the village of Mielniki while the southern column reached the forests east of Szack (Shatsk in Ukrainian). Polish reconnaissance patrols reported that Szack was occupied by Soviet infantry and tanks, and Orlik-Rueckemann ordered his two columns to form a defensive line along the edge of the forest and provoked a Soviet attack.

At 08.00, a Soviet tank unit composed mostly of T-26 light tanks began a direct assault on the Polish positions, and the Poles held their fire until the tanks had reached short range. When the tanks were only some 545 yards (500 m) from their line, the Poles opened fire with their 37-mm Bofors wz.36 anti-tank guns. The fire of these weapons was soon complemented by that of the infantry and 2.95-in (75-mm) field guns. All the Soviet tanks were destroyed and Major Balcerzak’s battalion was ordered to attack Szack. The Soviet units were taken by surprise and, after a short hand-to-hand fight, were routed. Only a small part of the Soviet motorised infantry managed to retreat, but even so had to abandon all their trucks, artillery and nine T-26 tanks. The Poles also captured the staff headquarters.

According to the orders of General Major Ivan N. Russyanov’s Soviet 52^nd Division found in the captured headquarters, the Soviet units operating from the Kobryń area were to 'clean up the area to the east of Bug river from the bands of Polish officers'. At 14.00. the Soviet reserve units appeared in the area and Orlik-Rueckemann decided to withdraw his troops into the forests.

The Polish forces withdrew unopposed and resumed their movement toward the Bug river. The only Polish unit destroyed by the Soviets was the protected wagon base of the Korpus Ochrony Pogranicza’s 'Polesie' Brigade, which was taken by surprise near the village of Mielnik by armoured forces of Komdiv Vasili I. Chuikov’s 4^th Army. After a brief fight, the Polish unit surrendered, and all its officers and non-commissioned officers, totalling osme 500 men, were then shot.

The remaining Polish forces evaded capture and crossed the Bug river, where they took part in the 'Battle of Wytyczno'.