The unit received weapons, technical equipment and rolling stock from the 225th Reserve Infantry Regiment.

[2] In May of that year, the unit was renamed the 13th Polish Rifle Regiment and was included in the army of General Józef Haller.

The 18th Infantry Division, which was part of it, was to organize defense on the Narew - Biebrza river line and take positions between Ostrołęka and Łomża.

[9][10] Emergency mobilization in the regiment began on August 24, 1939, at 6:00 a.m. in the barracks in Białystok and facilities in the city.

[11] Reservists were sent to the baths, underwent a medical examination, and then were uniformed and received equipment, weapons and ammunition.

The attendance of privates was exemplary, and some reserve officers reached the regiment with a slight delay.

On the night of August 26/27, the fully mobilized 9th Rifle Company loaded into wagons and left for Ostrołęka, from where it moved to Łodziska to ensure the concentration of the regiment.

[14] On August 31, the regiment's positions were inspected by the divisional infantry commander, Col. Aleksander Hertel.

[16] The main forces of the 42nd Infantry Regiment were grouped on the Narew line, with the 3rd battalion advanced in the foreground in the area of Łodziska.

Due to the threat to Białystok from approaching Wehrmacht, the marching battalion of the 42nd Infantry Regiment was subordinated to the city defense command, which was held by Lt. Col. Zygmunt Szafranowski, and the chief of staff was Capt.

The 2nd rifle company with the marching platoon of the 14th horse artillery division (two guns) took up positions on Wysoki Stoczek.

On September 14, the Lötzen Infantry Brigade attacked the positions of Lieutenant Kiewlicz's assembly company in the village of Żółtki.

On September 15, around 9:00 a.m., along the road from Żółtki, units of "Lötzen" Brigade approached the defensive positions of the 2nd Rifle Company.

With strong support from artillery, aviation and armoured cars, it was pushed from Wysoki Stoczek.

cadet Antoni Malecki stopped the further attack of the German infantry and drove him away from Wysoki Stoczek.

On September 17, the marching battalion of the 42nd Infantry Regiment was bombed by the Luftwaffe, then traveled by rail through Lida to Vilnius in order to strengthen the organized defense of the city.

[19] While the transport with the battalion was stopping at the railway station in Bieniakonie (in today's Belarus), information came about the entry of Red Army.

On the morning of September 19, after reaching the Landwarowo railway station, the battalion received information about the occupation of Vilnius by Soviet troops.