In mid-September 1939, Division Kobryn was incorporated into Independent Operational Group Polesie, commanded by General Franciszek Kleeberg.
It entered action on September 14, when it clashed with advancing Wehrmacht along the road from Brzesc nad Bugiem to Kobryn.
On September 18, it fought the Germans in Kobryn, and retreated southwest, to join the Polesie Group.
It took part in all battles and skirmishes of Independent Operational Group Polesie: on September 28–30, it defeated the Red Army near Jablon and Milanow, on October 1–2 it fought the Wehrmacht near Serokomla.
On October 3, 1939, the division entered forests north of Wola Gułowska, and two days later, reinforced by Cavalry Brigade Edward (Colonel Edward Milewski), it attacked German positions in Wola Gułowska and Helenów.