During the Polish mobilization of 1939, prior to the invasion of Poland, the division was mobilized and dispatched to the area west of Ciechanów, where it was to form a strategic reserve of the Polish Modlin Army defending the Modlin Fortress and the northern approaches to Warsaw. Commanded by Col. Teodor Wyrwa-Furgalski, the unit was to enter combat should the Germans break through the Polish lines. However, already on September 2, the division was dispatched to the area of Mława, where it was to support the Polish units fighting in the Battle of Mława.