Shortly before the war, a decision was made to strengthen the Polish defences by construction of a line of field fortifications and concrete bunkers to the north of Mława, in the centre of the army's positions.

To ease the delaying actions in case of a war with Germany, the Polish General Staff decided that the Modlin Army should be transported to the border with East Prussia and should defend the line for as long as possible.

After the Polish secret mobilization had started in March 1939, the 20th Infantry Division was assigned to the Modlin Army and transported to the area of Mława.

At noon on 1 September 1939 the Polish line of defence manned by the 20th Infantry Division was attacked by the 1st Army Corps under General Walter Petzel.

[2] The following afternoon the German units started heavy artillery bombardment of the Polish positions, in coordination with an aerial attack by Ju 87 Stukas of Sturzkampfgeschwader 1.

Sensing weakness on the Polish right flank, von Küchler ordered Panzer Division Kempf moved towards Rzęgnowo to prepare for an assault the following day.

As the Mazowiecka Cavalry Brigade operating further eastwards was also endangered by German armoured troops, the army commander ordered the division to split its forces and attack in two directions: towards Grudusk east of Mława and towards Przasnysz.

The Germans used the local civilians as human shields,[5][6][7] which allowed them to finally capture several bunkers on the left flank of the Polish forces, but were unable to push forwards.

On the right flank, in the Rzegnów section of the front to the east of the swamps, the attacks were more successful and in the late evening elements of German Wodrig Corps finally broke through the lines of the 79th Infantry Regiment to the rear of the Poles.

At 09:00 on September 4, General Emil Krukowicz-Przedrzymirski, facing the risk of his forces being outflanked and surrounded, ordered the 20th division and the remnants of the 8th to withdraw towards Warsaw and Modlin, finally abandoning the fortified positions.

Although the position was abandoned, the German forces suffered substantial losses and it was not until 13 September that they finally managed to reach the Modlin Fortress, located less than 100 kilometres to the south.