German units were to invade Poland from three directions: All three assaults were to converge on Warsaw, while the main Polish army was to be encircled and destroyed west of the Vistula.

[9] At 08:00 on 1 September, German troops, still without a formal declaration of war being issued, attacked near the Polish town of Mokra, and the Battle of the Border had begun.

Parts of it, under Admiral Józef Unrug, would continue to defend pockets of the coast over the next few days or weeks (at the battles of Westerplatte, Gdynia, Hel and others), but the rest was forced, together the Army Poznań under Tadeusz Kutrzeba, to retreat east from their defensive lines in Greater Poland towards Kłodawa, in Kujawy.

The Polish forces retreated towards their secondary lines of defence at the Vistula and Narew rivers, which allowed the Germans to move towards their main objective, Warsaw.

By 6 September, Polish forces were in retreat, and Marshal of Poland Edward Rydz-Śmigły ordered all troops to fall back to the secondary lines of defences at the vistula and San rivers.

Those defeats, in turn, made it more difficult for the Polish forces to fall back in an organised way to the secondary lines of defence (behind the Vistula and near the Romanian Bridgehead).