On 15 and 16 September, Army Pomorze took up defensive positions on the north bank of the Bzura. General Stanisław Grzmot-Skotnicki's group was 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 destroy the Polish forces, the Germans used most of their 10th Army, including two armoured, one motorized, and three light divisions, 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 Poles were forced out of Sochaczew, a town on the Bzura river, and 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. Polish forces were forced to abandon most of their heavy equipment while crossing 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.