In the summer of 1939 it was moved west and became part of the
Modlin Army. Its task was to defend the partly fortified area north of
Mława, near the interwar border of Poland and
East Prussia. Between September 1 and 3, 1939 (see:
September Campaign), the division defended positions around Mława in the
Battle of Mława, facing the more numerous and better equipped units of the German
I Army Corps (composed of:
11th Infantry Division,
61st Infantry Division and the
Panzer Division Kempf). The
Wehrmacht was advancing southwards, towards
Warsaw, but first attacks were repulsed with the loss of around 25 German tanks. Unable to capture Mława in frontal attack, the Germans decided to go around and attack from sides. As they threatened 20th I.D.'s rear, the Poles were ordered to withdraw. However, supported by the neighbouring Polish
8th Infantry Division, the defenders kept their positions. Facing the difficult situation, the 20th I.D. was ordered to withdraw on the night of September 3–4. Bombed by the Germans, the division around September 7 reached the area of
Płock. Then, the unit's remains entered Warsaw, helping with its defence from northwest. They capitulated together with the Warsaw garrison on September 28 (see:
Siege of Warsaw (1939)). The Germans who fought against the 20. at Mława gave it a nickname "
Steel Division." == Order of Battle (1939) ==