Prusy Army The Prusy Army ( ) was one of the Polish armies to fight during the Invasion of Poland in 1939 . Created in the summer of 1939 as the main reserve of the Commander in Chief , it was commanded by Gen . Stefan Dąb - Biernacki . The word " Prusy " in the Polish language means Prussia , but this name only served as a codename and the region of operations of this army was far from East Prussia . This is in contrast to other Polish armies in 1939 which were named after the geographical regions where they formed . The Prusy Army , whose original name was Warszawa Army , was named so after a folwark called Prusy , which served as the headquarters of General Dąb - Biernacki . Tasks . According to the " Plan West " ( " Plan Zachód " , code name for the Polish mobilization plan ) it was to be composed of units mobilized as the second and third waves , and its main purpose was to cooperate with the nearby armies " Łódź " and " Kraków " . It was being mobilized in two groups after the outbreak of hostilities . It consisted of 6 infantry divisions , 1 cavalry brigade and a battalion of tanks . Once ready , it was supposed to stop the main German thrust . The army , divided into two groups , was concentrated southwest of Warsaw ( in the triangle between Kielce , Radom , and Tomaszów Mazowiecki ) , as Marshall Edward Śmigły - Rydz was convinced that main German thrust would be directed at Warsaw , via Łódź . The Polish military strategists correctly predicted that German forces would try to push in between the armies " Łódź " and " Krakow , " but they failed to predict the fast pace of the German advance . Therefore , in Polish plans " Armia Prusy " was to be fully mobilized only by the 14th of September 1939 , while in reality it needed to be ready as soon as the late night of September 3 , 1939 . As Polish historians Czesław Grzelak and Henryk Stańczyk wrote : " Polish planners miscalculated the speed of the advance of German panzer and motorized units , and plans for the conflict were based on the experiences of the First World War " . The Poles were surprised by how fast they would have to fall back to prevent destruction of their units . This miscalculation lead to the capture of the city of Łódź - a major setback to the Polish plan of defending the country west from the Vistula river at least until October 1939 . More surprising should be that the " Polish schedule " was followed by the French one year later ; they did refuse to read the report pointing out mistakes of the Polish military , and drawing conclusions on how to avoid them in the future . The report was prepared by the Polish generals and handed to the French , and the British in their respective languages , in late 1939 . Operational history . The northern group was to back up the Łódź Army near Łódź and Sieradz , while the southern group was to support Kraków Army by preparing the defence of central Vistula river area . Because of fast German advance both groups entered combat separately and most units did not reach full mobilization . Due to the deteriorating situation in the battle of the border , it was decided to push partially mobilized units of Prusy Army to the front as soon as possible . It must be noted that in original Polish plans , the 10th Motorized Cavalry Brigade belonged to the Prusy Army . In late July 1939 , Marshall Śmigły - Rydz decided to move this unit to Kraków Army . The northern group , seriously damaged in the battles of Piotrków and Tomaszów Mazowiecki ( September 5 - September 6 ) was forced to cross the Vistula . The southern group , much less organized and poorly commanded , fought in the Battle of Iłża , after which it was cut off from the Vistula , encircled near Radom and destroyed ( September 8 – 9 ) . Units of the Army which retained cohesion or broke through the German lines later fought in the defense of Warsaw or joined the other Armies in the northern - central Poland . Organization . The army was commanded by General Stefan Dąb - Biernacki ; his chief of staff was Tomasz Obertyński . Biernacki was in direct command of the Northern Group ; the Southern Group was commanded by General Stanisław Skwarczyński .