Complex sentence: After Wilno was overrun by the Bolsheviks, Pilecki and his unit conducted partisan warfare behind Russian lines, eventually escaping to Białystok where Pilecki enlisted the newly-eatablished Polish Army.
Split into simpler sentences: After Wilno was overrun by the Bolsheviks, Pilecki and his unit conducted partisan warfare behind Russian lines. They eventually escaped to Białystok where Pilecki enlisted the newly-established Polish Army.