Polish forces were stretched thinly along the Polish-German border and lacked compact defence lines and good defence positions along disadvantageous terrain. That strategy also left supply lines poorly protected. One third of Poland's forces were massed in or near the Polish Corridor, making them vulnerable to a double envelopment from East Prussia and the west. Another third were concentrated in the north-central part of the country, between the major cities of Łódź and Warsaw. The forward positioning of Polish forces vastly increased the difficulty of carrying out strategic maneuvres, compounded by inadequate mobility, as Polish units often lacked the ability to retreat from their defensive positions, as they were being overrun by more-mobile German mechanized formations.