the retreat route of Polish forces. By 8 September, advanced German troops reached Warsaw, which marked the beginning of the Siege of Warsaw. At the same time, German forces had lost contact with Army Poznań, and the German command assumed that the army must have been transported by rail to aid Warsaw's defence. They were unaware that in fact Army Poznań had merged forces with Army Pomorze, which they considered, since its defeat at Bory Tucholskie, to be no longer a significant threat. On 8 September, the Germans were certain that they had eliminated major Polish resistance west of Vistula and so prepared to cross it and to engage the Polish forces on the other side. Meanwhile, General Kutrzeba and his staff officers had suspected, even before the German invasion, that it would be the neighbouring armies that would bear the German attack and so they had developed plans at an offensive to the south to relieve Army Łódź. During the first week of the campaign, however, those plans were rejected by the Polish commander-in-chief, Marshal Edward Rydz-Śmigły. By 8 September, Kutrzeba had lost contact with Rydz-Śmigły, who had relocated his command center from Warsaw to Brest. Those factors made Kutrzeba decide to go forward with his plan. His situation was dire, as German forces were close to surrounding his units: the German 8th Army had secured the southern bank of the [[Bzriver, and the German 4th Army had secured the northern bank of Vistula, from Włocławek to Wyszogród, and its elements were attacking the rear of the Armies Pomorze and Poznań from the direction of Inowrocław and crossing the Vistula near Płock. Opposing forces The Polish forces consisted of Army Poznań and Army Pomorze. The German forces included the 8th Army under Johannes Blaskowitz and 10th Army under Walther von Reichenau of Army Group South (Heeresgruppe Süd), elements of the 4th Army under Günther von Kluge of the Army Group North (Heeresgruppe Nord) and air support (Luftflotte 1 and Luftflotte 4). Battle The battle can be divided into three phases: Phase I — Polish offensive towards Stryków, aiming at the flank of the German 10th Army (9–12 September) Phase II — Polish offensive towards Łowicz (13–15 September) Phase III — German counterattack and eventual defeat of the Poles, who withdraw towards Warsaw and Modlin (16–19 September) On the night of 9 September, the Polish Poznań Army commenced a counterattack