Battle of Mława
At noon on September 1, 1939 the Polish line of defence manned by the 20th Infantry Division was attacked by the 1st Army Corps under General Walter Petzel.
Although the attacking forces were equipped with tanks and supported by warplanes, the initial assault was repelled by Polish-made 37mm Armata ppanc. wz. 36 anti-tank guns. Georg von Küchler, the commander of German Third Army, ordered his units to attack the Polish forces several times in a row, but all attacks were broken and in the late evening the Germans were forced to withdraw to their initial positions.
The following afternoon the German units started heavy artillery bombardment of the Rzegnów position on the right flank of the Polish forces. After two hours of constant artillery fire, the assault was started and, in the result of close combat, the Polish defenders started to waver. The counterattack of the Polish 79th Infantry Regiment was unsuccessful and the commander of the Polish Modlin Army ordered the 20th Division to extend further eastwards and prepare the defence of its right flank between the villages of Dębsk and Nosarzewo. At the same time the 8th Infantry Division, until then held in reserve near Ciechanów, was ordered to prepare a counterattack.
The 8th Division arrived in the area in the early hours of September 3. As the Mazovian 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. However, conflicting orders and German diversants operating in the rear disrupted both attacks and led to chaos in the Polish ranks. In the evening the division was mostly destroyed and only the 21st Infantry Regiment of Colonel (later General) Stanisław Sosabowski managed to withdraw from the fights towards the Modlin Fortress. Despite this, the German attacks towards both flanks of the 20th Infantry Division were unsuccessful.
On September 3 the German engineers finally managed to cut through Polish antitank barriers. The Germans used the local civilians as human shields, 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. This widened the front gap in the area of Grudusk.
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.
On September 1st and 2nd, Polish Army was already involved in heavy fighting (e.g. Battle of Mlawa, Battle of Pomerania), while retreating eastwards in attempt to establish line of defence and delay the invaders. On September 3rd, German forces cut the "Polish Corridor". On the same day, England and France declared war on Germany but didn't take any serious action to directly help Poland. Polish forces made up of foot infantry were unable to fallback and establish defensive lines being constantly chased and outpaced by German motorized and Panzer units. Many Polish units and even armies were surrounded and destroyed, while attempting to either defend or withdraw. Polish High Command realized as early as September 5th that the situation was critical and there was no hope without immediate help of its Allies. Many units were cut-off but continued fighting.