Just an historical overview, taken generally from the Wiki pages:
After the first 10 days of the German invasion of Poland, elements of at least three Polish armies had retreated to and converged on the area north of Lodz and east of Warsaw. The River Bzura was a natural defensive line north of Lodz. The Lodz Army had drawn up plans for a counter-attack south, to forestall the expected German envelopment. The Polish supreme command had refused to sanction the counterattack, wanting the armies to fall back on Warsaw. However, when High Command HQ transferred out of Warsaw and communications faltered, the commanders on the spot north of the Bzura took matters into their own hands and launched their offensive.
10 days of heavy fighting followed which resulted in a German victory and envelopment, but laid the foundation for many a tale of valor in the face of a seemingly hopeless cause.
