Soon after the outbreak of the Polish January 1863 Uprising against Imperial Russia, 15-year-old Prus ran away from school to join the insurgents. He may have been influenced by his brother Leon, one of the Uprising's leaders. Leon, during a June 1863 mission to Wilno (now Vilnius) in Lithuania for the Polish insurgent government, developed a debilitating mental illness that would end only with his death in 1907.