The 'Battle of the Tuchola Forest' was a battle fought between German and Polish forces within the context of the 'Battle of the Border', which was the first stage of the German 'Weiss' (i) invasion of Poland (1/5 September 1939).

The German strategic plan was to invade Poland in two major groupings in the south and north. The southern grouping was Generaloberst Gerd von Rundstedt’s Heeresgruppe 'Süd', which was to advance to the east and then the north-east, and the northern grouping was Generaloberst Fedor von Bock’s Heeresgruppe 'Nord', which was to advance to the east and with the support of forces from East Prussia, the south-east: the two army groups were thus to meet in the area to the east of Warsaw, the Polish capital, and thereby complete a strategic envelopment to trap Poland’s armies and force Poland’s capitulation. One of the first tasks of Heeresgruppe 'Nord' was to drive its 4^th Army, under the command of General Günther von Kluge, from eastern Pomerania to link with the 3^rd Army, under the command of General Georg von Küchler, advancing to the south from East Prussia. Thus 4^th Army's advance would eliminate the so-called 'Polish corridor', the narrow strip of land provided Poland’s only access to the Baltic Sea between eastern Pomaniania and the Free City of Danzig.

Starting on the first day of the German invasion, the 'Battle of the Tuchola Forest' lasted five days and ended in a significant German victory. As a result of poor Polish command and control, as well as their numerical, technical and tactical superiority, the Germans crippled the Armia 'Pomorze' and, by breaking through the 'Polish corridor', to connect mainland Germany with East Prussia.

The battle was fought against the judgment of Generał dywizji Władysław Bortnowski, commander of the Armia 'Pomorze', who believed that the 'Polish corridor' was a very poor defensive position and had repeatedly asked for permission to withdraw his forces from it.

The Tuchola forest lay in 'Polish corridor', and was a large area, and the difficulty of its terrain was thought by the Polish high command to be a good defensive position. However, the Germans had held their Truppenübungsplatzgruppe military exercises in the area until 1919, and were therefore familiar with it, and furthermore General Heinz Guderian, commander of the XIX Corps, the 4^th Army's spearhead, had been born in nearby Chełmno. The 4^th Army comprised the XIX Corps and General Adolf Strauss’s II Corps based in Pomerania immediately to the west of the 'Polish corridor' corridor.

The XIX Corps comprised Generalleutnant Paul Bader’s 2^nd Division (mot.), Generalleutnant Mauritz von Wiktorin’s 20^th Division (mot.) and Generalleutnant Leo Geyr von Schweppenburg’s 3^rd Panzerdivision. The II Corps comprised Generalleutnant Walter Lichel’s 3^rd Division and Generalleutnant Franz Böhme’s 32^nd Division.

The Polish forces in this initial northern theatre were elements of the Armia 'Pomorze', namely Pulkownik Józef Werobej’s 9^th Division, Generał brygady Juliusz Drapella’s 27^th Division, and Generał dywizji Stanisław Grzmot-Skotnicki’s Operational Group 'Czersk'.

Despite several Polish tactical successes, including the cavalry charge at Krojanty on 1 September, and the destruction of a German armoured train near Chojnice, the Germans were able to advance quickly. Not all of the Polish forces were in position on 1 September, and the speed of the German advance generated confusion in the Polish ranks. In addition, communications problems prevented cohesive action by the scattered Polish forces. The Poles were soon forced to abandon plans for a counter-offensive and thus retreated, pursued by more mobile German armoured and motorised formations. The German forces had a significant quantity of armoured support, including more than 300 tanks under Guderian’s command.

Most of he Polish formations and units had been surrounded by 3 September. Some were destroyed, while others managed to break through toward Bydgoszcz.

By 5 September, the Germans had largely completed their seizure of the 'Polish corridor'. At that point some of the German forces moved to mop up isolated pockets of Polish resistance farther to the north in fortified areas on the coast of the Baltic Sea, while others continued their push to the south-east, and therefore still deeper into Polish territory.

About two-thirds of Armia 'Pomorze' survived the battle and came under the command of the Armia 'Poznań', with which they fought in the 'Battle of the Bzura'.