During the German invasion which began on 1 September 1939, Sosnkowski proposed forming a group of armies in the region of Warsaw and Kutno, in order to tie-up the German forces there. His ideas were rejected by Rydz-Śmigły, which may have had resulted in uncoordinated military activity and eventually the Polish defeat in the Battle of the Bzura. Only on 11 September he was appointed commander of the Southern Front. Retreating toward Lwów, he conducted several victorious engagements with German forces. The Soviet invasion of 17 September made further withdrawal in the south-eastern direction impossible and Sosnkowski ordered his army, defeated by 22 September, to disperse. Disguised, he crossed the Soviet occupied territory and reached Hungary.