The Polish plan for defense against the German invasion, Plan West, called for the defense of the borders.
This was dictated more by political than military concerns, as Poles feared that the Germans, after taking over territories they lost in the Treaty of Versailles, would try to end the war and keep those territories.
While defending the borders was riskier, the Poles were counting on the British and French counteroffensive (that never came).
Due to this, Army Pomorze under general Władysław Bortnowski found itself in the Polish Corridor, surrounded by German forces on two fronts, and Army Poznań under general Tadeusz Kutrzeba was pushed to the westernmost fringes of the Second Polish Republic, separated both from its primary defensive positions, and from other Polish Armies.

The German offensive proved the folly of the border defense plan in the first days of the war.
Army Pomorze was defeated in the battle of Bory Tucholskie, and forced to retreat towards the south-east.
Army Poznań, meanwhile, although not facing heavy German assaults, was forced to retreat east due to defeats of its neighbours (Army Pomorze in the north and Army Łódź in the south); both of them were retreating, meaning that Army Poznań was in danger of being flanked and surrounded by the German forces.
On 4 September, Army Poznań moved through Poznań and abandoned it to the enemy, although at this point it was not in contact with any significant German forces.
By 6 September, Armies Pomorze and Poznań had linked, forming the strongest Polish operational unit in the campaign, and general Bortkowski accepted the command of general Kutrzeba.

On 7 September, Polish forces became aware of the German push towards Łęczyca, which if successful could cut off the retreat route of Polish forces.
By 8 September, advanced German troops reached Warsaw, marking the beginning of the 1939 siege of Warsaw.
At the same time, German forces had lost contact with Army Poznań, and German command assumed that the army must have been transported by rail to aid Warsaw's defense; they were unaware that in fact Army Poznań had merged forces with Army Pomorze, which they considered, since its defeat at Bory Tucholskie, no longer a significant threat.
On 8 September the Germans were certain that they had eliminated major Polish resistance west of Vistula and were preparing to cross it and engage the Polish forces on the other side.

Meanwhile, general Kutrzeba and his staff officers had suspected, even before the German invasion, that it would be the neighbouring Armies that would bear the German attack, and had developed plans at an offensive towards the south, to relieve Army Łódź.
In the first week of the campaign, those plans, however, were rejected by the Polish commander-in-chief, Marshal Edward Rydz-Śmigły.
By 8 September Kutrzeba had lost contact with Rydz-Śmigły, who had relocated his command center from Warsaw to Brest; due to these factors, Kutrzeba decided to go forward with his plan.
His situation was dire, as German forces were close to surrounding his units: the German 8th Army had secured the southern bank of the Bzura river and the German 4th Army had secured the northern bank of Vistula, from Włocławek to Wyszogród, and its elements were attacking the rear of the Armies Pomorze and Poznań from the direction of Inowrocław and crossing the Vistula river near Płock.
The Polish plan for defense against the German invasion, Plan West, called for the defense of the borders. This was dictated more by political than military concerns, as Poles feared that the Germans, after taking over territories they lost in the Treaty of Versailles, would try to end the war and keep those territories. While defending the borders was riskier, the Poles were counting on the British and French counteroffensive (that never came). Due to this, Army Pomorze under general Władysław Bortnowski found itself in the Polish Corridor, surrounded by German forces on two fronts, and Army Poznań under general Tadeusz Kutrzeba was pushed to the westernmost fringes of the Second Polish Republic, separated both from its primary defensive positions, and from other Polish Armies.
The German offensive proved the folly of the border defense plan in the first days of the war. Army Pomorze was defeated in the battle of Bory Tucholskie, and forced to retreat towards the south-east. Army Poznań, meanwhile, although not facing heavy German assaults, was forced to retreat east due to defeats of its neighbours (Army Pomorze in the north and Army Łódź in the south); both of them were retreating, meaning that Army Poznań was in danger of being flanked and surrounded by the German forces. On 4 September, Army Poznań moved through Poznań and abandoned it to the enemy, although at this point it was not in contact with any significant German forces. By 6 September, Armies Pomorze and Poznań had linked, forming the strongest Polish operational unit in the campaign, and general Bortkowski accepted the command of general Kutrzeba.
On 7 September, Polish forces became aware of the German push towards Łęczyca, which if successful could cut off the retreat route of Polish forces. By 8 September, advanced German troops reached Warsaw, marking the beginning of the 1939 siege of Warsaw. At the same time, German forces had lost contact with Army Poznań, and German command assumed that the army must have been transported by rail to aid Warsaw's defense; they were unaware that in fact Army Poznań had merged forces with Army Pomorze, which they considered, since its defeat at Bory Tucholskie, no longer a significant threat. On 8 September the Germans were certain that they had eliminated major Polish resistance west of Vistula and were preparing to cross it and engage the Polish forces on the other side.
Meanwhile, general Kutrzeba and his staff officers had suspected, even before the German invasion, that it would be the neighbouring Armies that would bear the German attack, and had developed plans at an offensive towards the south, to relieve Army Łódź. In the first week of the campaign, those plans, however, were rejected by the Polish commander-in-chief, Marshal Edward Rydz-Śmigły. By 8 September Kutrzeba had lost contact with Rydz-Śmigły, who had relocated his command center from Warsaw to Brest; due to these factors, Kutrzeba decided to go forward with his plan. His situation was dire, as German forces were close to surrounding his units: the German 8th Army had secured the southern bank of the Bzura river and the German 4th Army had secured the northern bank of Vistula, from Włocławek to Wyszogród, and its elements were attacking the rear of the Armies Pomorze and Poznań from the direction of Inowrocław and crossing the Vistula river near Płock.
Battle of the Bzura