At the outbreak of the German invasion of Poland in 1939 Sławik joined the Polish mobilised police battalion attached to the Kraków Army.
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In an attempt to outflank the positions of the Polish Kraków Army under Gen. Antoni Szylling defending Silesia and western Lesser Poland, the Germans crossed the Tatra passes and assaulted the towns of Chabówka and Nowy Targ.
Backed by the Slovakian units (see Slovak invasion of Poland (1939)), German divisions managed to break through Polish positions near Tymbark and Limanowa, causing a gap between Karpaty Army and the retreating Kraków Army.