The Brigade was ordered to take part in the fighting near Krosno, Dukla and Sanok. When on 8 September 1939 the Sloviakian troops entered Rymanów and German troops entered Krosno, the Brigade began to withdraw towards Stryj and Sambor. However, when the news came that the Soviet troops had entered Poland from the east, it stopped any further action. Some units crossed the Hungarian border, where they laid down their arms. This is how Mycielski recalled that period in September 1979:
40 years ago – (18 September?) – I was crossing the Uzhok Pass in the Carpathians with the entire company – we gave our weapons to the Hungarians – some Hungarian general stood ‘to attention’ and cried, some non-commissioned Polish officer shot himself a few steps away from us, near the forest. A Polish scene, and Polish scenery” (9 September 1979, in Z. Mycielski, "Notatki od 29 VIII 79 – 12 II 80", unpublished manuscript, Zygmunt Mycielski Archive).
Mycielski himself managed to get to Budapest, where he went straight to his cousin, Janoš Eszterházy. Eszterházy helped him to get, via Yugoslavia and northern Italy, to France, where Mycielski wanted to joins Polish units being formed in the French army.