The 'Battle of Mława' was a battle between German and Polish forces, within the context of the 'Frontier Campaign' of 'Weiss' (i), in the area to the north of the town of Mława in northern Poland (1/3 September 1939).

This was one of the opening battles of 'Weiss' (i) and this of the whole of World War II, and involved the forces of Generał brygady Emil Krukowicz-Przedrzymirsky’s 'Modlin' Army and General Georg von Küchler’s 3^rd Army. 

Under the terms of the Treaty of Versailles, which marked the end of World War I, a new border was created between Germany and Poland only some 75 miles (120 km) to the north of Warsaw, the capital city of Poland, a newly created state independent of Russia. In 1939 the 'Modlin' Army, under the command of Krukowicz-Przedrzymirski, was the main defensive force guarding the Polish borders in the north. It was located along the border with East Prussia, an organic part of Germany, and was to stop German forces from any advance on Warsaw, and was centred on the fortress of Modlin. Shortly before the start of 'Weiss' (i), the Poles decided to strengthen their 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.

The 'Modlin' Army’s main line of defence was located along the line of Narew and Vistula rivers in an area comparatively thickly scattered with 19^th-century fortifications, but the plains to the north of it were almost defenceless and indeed indefensible. To facilitate the fighting of delaying actions in the event of war with Germany, the Polish general staff decided that the 'Modlin' Army should be redeployed to the border with East Prussia, which it should hold for as long as possible. After this, Przedrzymirski-Krukowicz’s formations and units were to withdraw to the south and defend the line of the Narew and Vistula rivers in co-operation with the forces of the 'Narew' Independent Operational Group.

After the start of the Polish secret mobilisation in March 1939, General brygady Wilhelm Lawicz-Liszka’s 20^th Division was assigned to the 'Modlin' Army and transported into the area of Mława. In addition, the army commander was assigned a number of trainloads of concrete and other construction materials, together with several combat engineering battalions, as it had been decided that a line of fortifications was to be constructed in the area held by that division. On 19 June the project was ready and was finally approved by Marszałek Polski Edward Rydz-Śmigły, the Polish commander-in-chief, on 3 July. The new line of trenches and concrete bunkers, strengthened with anti-tank ditches and obstacles, was to be constructed along a low glacial hill overlooking the valley of the Mławka river in the area to the north of the town. The flow of the river’s water could be blocked by a dam to enhance the defensive capability of the area. In the centre, the swampy terrain of the Niemyje marshes was virtually impassable by armoured vehicles. This swamp divided the overall area into two separate flanks. The western section was to be reinforced with 68 concrete bunkers while the eastern, much shorter, section was to have 25 such bunkers.

The peacetime base of the 20^th Division was Baranowicze. In case of a war with the USSR, it was planned as the first-line unit to defend a line of German fortifications of World War I vintage built in 1915. Thus most of the division’s men had been trained for the defence of fortified positions.

The construction of bunkers in the western section of the front, near Mława, began on 14 July, and was carried out mostly by the soldiers themselves, under the supervision of Major Juliusz Levittoux’s 20^th Engineering Battalion. The construction of the eastern flank bunkers near the village of Rzęgnowo started on 12 August. The soldiers were soon joined by a number of civilian volunteers, who helped to dig trenches. However, the positions and many of the bunklers had not been completed before the outbreak of war.

At 12.00 on 1 September, the Polish defences manned by the 20^th Division were attacked by General Walter Petzel’s I Corps of von Küchler’s 3^rd Army within Generaloberst Fedor von Bock’s Heeresgruppe 'Nord'. Although the attacking forces were equipped with tanks and supported by warplanes, the initial assault was repelled by Polish 37-mm anti-tank guns. von Küchler ordered his formations and units to attack the Polish forces several times, but all of the attacks were broken and late in the evening the Germans were forced to withdraw to their initial positions.

During the afternoon of the following day, the Germans started heavy artillery bombardment of the Rzegnów position on the Polish forces' right flank. After two hours of constant artillery fire, the assault was started and, in the resulting close-quarter combat the Polish defenders started to waver. The counterattack of the 79^th Regiment was unsuccessful and the 'Modlin' Army’s commander ordered the 20^th Division to extend itself farther to the east and prepare the defence of its right flank between the villages of Dębsk and Nosarzewo. At the same time Podpułkownik Teodor Furgalsky’s 8^th Division, until then held in reserve near Ciechanów, was ordered to ready itself for a counterattack.

The 8^th Division arrived in the relevant area during the early hours of 3 September. As the 'Mazovian' Cavalry Brigade operating farther to the east was also endangered by German armoured troops, the army commander ordered the division to split its forces and attack in two directions: toward Grudusk east of Mława and toward Przasnysz. However, conflicting orders and German saboteurs operating in the rear disrupted both attacks, and this led to chaos in the Polish ranks. In the evening the division was mostly destroyed and only Pułkownik Stanisław Sosabowsky’s 21^st Regiment managed to withdraw intact toward the fortress of Modlin. Despite this, the German attacks on both of the 20^th Division’s flanks were unsuccessful.

On 3 September the German engineers finally managed to cut through Polish anti-tank barriers. Using local civilians as human shields, the Germans finally captured several bunkers on the left flank of the Polish forces, but were unable to push farther forward. On the right flank, in the Rzegnów section of the front to the east of the swamps, the attacks were more successful and late in the evening elements of General Albert Wodrig’s Korps 'Wodrig'of two infantry divisions broke through the lines of the 79^th Regiment into the Polish rear area. This widened the gap in the front in the area of Grudusk.

Faced with the risk of his forces being outflanked and surrounded, Krukowicz-Przedrzymirski ordered the 20^th Division and the remnants of the 8^th Division to withdraw toward Warsaw and Modlin, finally abandoning the fortified positions. The withdrawal started early in the morning of 4 September. Although the German mechanised units suffered heavy losses and were unable to maintain any pursuit, the area to the south of Mława was very lightly forested and the Polish forces were constantly bombarded by the German artillery and strafed by the German air force, suffering heavy losses in both men and equipment.

Although the position had been 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 some 60 miles (100 km) to the south.