The division was fully assembled on September 10, 1939. It was ordered to defend the Vistula River line between the towns of Dęblin and Kazimierz Dolny. Due to General Olbrycht's sickness, it was "de facto" commanded by Colonel Bronisław Duch. On September 14, the 39th, which had become part of the Lublin Army, withdrew towards Chełm. Five days later, it engaged the Wehrmacht in Krasnystaw, then fought the German 4th Light Division near Zamość. The division took part in the Battle of Tomaszów Lubelski and capitulated on September 26, 1939.