He was graduate and later Professor of the Jagiellonian University until the Chair of Universal History was abolished by administrative decision of the Minister of Education from the Piłsudski camp Colonel Wacław Jędrzejewicz. That decision was considered a retaliation for Sobieski’s attempts to demonstrate that the success of the Polish army in the battle of Warsaw against the Bolsheviks (1920) was more the result of plans and decisions by commanders other than Józef Piłsudski. As a historian, he was associated with the National Democrats, and was a member of the National League and of the National Democratic Party. He wrote, inter alia, Trybun ludu szlacheckiego [“Champion of the nobility”] (1905), Polska a hugenoci po nocy św. Bartłomieja [“Poland and the Huguenots after St. Bartholomew’s Day”] (1910), Żółkiewski na Kremlu [“Żółkiewski in the Kremlin”] (1920) and Dzieje Polski [“History of Poland”], vols. 1–3 (1923–1925).