Pilecki was mobilized as a cavalry platoon commander on 26 August 1939. He was assigned to the 19th Infantry Division under General Józef Kwaciszewski, part of the Polish Army Prusy and his unit took part in heavy fighting against the advancing Germans during the invasion of Poland. The 19th Division was almost completely destroyed following a clash with the German forces on the night of 5 to 6 September at the battle of Piotrków Trybunalski. Its remains were incorporated into the 41st Infantry Division, which was withdrawn to the southeast toward Lwów (now L'viv in Ukraine) and the Romanian bridgehead. In the 41st Division, Pilecki served as divisional second-in-command of its cavalry detachment, under Major Jan Włodarkiewicz. He and his men destroyed seven German tanks, shot down one aircraft, and destroyed two more on the ground. On 17 September, the Soviet Union invaded eastern Poland, which worsened the already desperate situation of the Polish forces. On 22 September, the 41st Division suffered a major defeat and capitulated. Włodarkiewicz and Pilecki were among the many soldiers who did not follow the order of Commander-in-Chief General Edward Śmigły-Rydz to retreat through Romania to France, instead opting to stay underground in Poland.