It was improvised when it became obvious that quickly advancing Wehrmacht armored and motorized units would reach the Vistula river line.
Lublin Army was tasked with defending the crossings of the Vistula river from Modlin (north of Warsaw) to Sandomierz in the south.
Polish planners wanted to reinforce it with elements of the Prusy Army, which after the Battle of Radom had retreated from German encirclement, and managed to cross the Vistula.
Under its pressure, Polish forces retreated towards Krasnik, where they merged with Krakow Army, and together fought in the Battle of Tomaszow Lubelski (September 17–20).
The Army was commanded by general Tadeusz Piskor; his chief of staff was colonel Jan Zawisza.