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.