Armeekorps was first deployed on 10 August 1939 in the Allenstein sub-district within Military District (Wehrkreis) I in East Prussia.

[3][4] At the beginning of the campaign, XXI Army Corps advanced from a position southwest of Osterode in East Prussia in a southwesterly direction towards Grudziądz and Chełmo, with the goal of closing the Danzig Corridor and to unite with the 4th Army's forces advancing into Poland from Pomerania.

The XXI Army Corps was supported in its advances on the Polish 16th Infantry Division in Grudziądz, which was protected on its right by the Polish 4th, by heavy Luftwaffe aerial attacks against 4th Infantry Divisions, taking pressure off the left flank of XXI Army Corps.

Although the corps inflicted heavy losses on the Polish formations, the 4th and 16th Infantry Divisions withdrew from the area in good order, surrendering the city to the Wehrmacht.

[1] In the meantime, the XIX Army Corps (Heinz Guderian) had transferred from Pomerania over East Prussia to the sector and was subsequently, to the detriment of the fighting strength of Group Falkenhorst, assigned control of the 10th Panzer Division and the East Prussian fortress units.

[1] In this function, it made contact with forces of the Red Army that entered Poland from the east with the Soviet invasion that started on 17 September.

[2] Falkenhorst was tasked with the execution of Operation Weserübung, the invasions of Denmark and Norway, by the German dictator Adolf Hitler on 21 February 1940.