The Polish defensive plan called for a strategy of encirclement. It would allow the Germans to advance in between two Polish Army groups in the line between Berlin and Warsaw-Lodz, and Armia Prusy would then move in and repulse the German spearhead, trapping it. For that to happen, Armia Prusy needed to be fully mobilized by 3 September. However, Polish military planners failed to foresee the speed of the German advance and assumed that Armia Prusy would need to be fully mobilized by 16 September.