It was not part of peacetime structure of the army, and was formed on August 24–27, 1939, out of units stationed at military districts I (Warsaw) and III (Grodno).
In the night of September 6/7, 1939, the division, together with other units of the Wyszków Group, attacked the town of Różan, in order to hold Polish positions along the Narew river.
After another retreat, the Division regrouped in the village of Piszczac, where it rested for two days, and received new equipment from military magazines at Biała Podlaska.
Even though General Rys-Trojanowski was not in the position to dispose of units of Modlin Army, Colonel Zieleniewski obeyed the order, and in the morning of September 15 commanded his soldiers to march to Brzesc.
On September 17, the Division took part in a skirmish near Włodawa, after which it joined Northern Front of General Stefan Dąb-Biernacki, and marched southwards.