September, General Władysław Bortnowski's 26th and 16th Infantry Divisions crossed the Bzura near Łowicz. The Polish 4th Infantry Division reached the road linking Łowicz with Głowno. At that point, however, Bortnowski ordered the 26th Infantry Division to retreat. He had learned of the withdrawal of the German 4th Panzer Division from the outskirts of Warsaw and was concerned that the Panzer division posed a threat to his men. On 15 and 16 September, Army Pomorze took up defensive positions on the north bank of the Bzura. General Stanisław Grzmot-Skotnicki's group between Kutno and Żychlin, General Michał Karaszewicz-Tokarzewski's units near Gąbin and parts of Army Poznań by the Bzura near Sochaczew were ready to begin their drive towards Warsaw. To encircle and to destroy the Polish forces, the Germans used most of their 10th Army, including two armoured, one motorized and three light divisions, which was equipped with some 800 tanks altogether. The attack from all sides on Polish positions started on 16 September with the support of the Luftwaffe. On 15 September, the Poles were forced out of Sochaczew, a town on the Bzura river, and were trapped in a triangle of Bzura, Vistula and German forces. The German 1st Panzer Division, after crossing the Bzura between Sochaczew and Brochów and engaging the Polish 25th Infantry Division, managed to capture Ruszki, but its advance was then halted. Poles began to cross the Bzura near the Vistula, north of Sochaczew, and retreat towards Warsaw. The Polish forces were forced to abandon most of their heavy equipment while they crossed the river. On 17 September, German heavy artillery was shelling the crossing north of Brochów, and the largest air operation of the campaign began, with the Luftwaffe attacking the retreating Polish forces. During the night of 17 September, the main forces of Army Poznań attacked the German forces to break out of the German encirclement between Witkowice and Sochaczew. The 15th Infantry Division and Podolska Cavalry Brigade again crossed the Bzura in Witkowice. In Brochow, the 25th and the 17th Infantry Divisions crossed the Bzura. The 14th Infantry Division was concentrated in Łaziska. At the same time, Army Pomorze marched towards the villages of Osmolin, Kierozia and Osiek. In the morning, the Germans started their drive towards the south along both banks of the Bzura and were supported by more than 300 aircraft and heavy artillery. German howitzers, taking advantage of their