Kraków. In 1915, Sławek was sent by Piłsudski to Warsaw, where he created local structures of Polish Military Organisation (POW). At the same time, he formed a secret body within the POW, called Military Association (Zwiazek Wojskowy), later renamed into Organization A. In December 1916, after the creation of Provisional Council of State, Sławek was employed by its Military Commission. Following the Oath crisis, he was arrested by the Germans (July 13, 1917), and sent to Warsaw Citadel and then to Szczypiorno and Modlin. He was finally released on November 12, 1918. Polish Army On January 1, 1919, Sławek joined the 4th (Intelligence) Department of Polish General Staff. In May 1919, he was sent to the Lithuanian borderland, where he tried to reach an agreement with the Lithuanian general Silvestras Zukauskas. The purpose was to initiate a joint Polish-Lithuanian anti-Bolshevik front, but his attempt failed. In November 1919, he left for Tallinn, to negotiate with envoys of Estonian and Latvian governments. In January 1920, Sławek was sent to Ukraine, where he cooperated with Civil Commissar for Podolia and Volhynia, Antoni Minkiewicz. Promoted to major (April 22, 1920), he was a Polish envoy to Ataman Symon Petliura. Together with Wacław Jędrzejewicz, he signed a military appendix to the Treaty of Warsaw (1920), between Poland and Ukrainian People's Republic. For the remaining part of Polish–Soviet War he remained in southeastern provinces of Poland, where he tried to create mixed, Polish-Ukrainian volunteer units. Between 1922 and 1923, Sławek attended the Wyższa Szkoła Wojenna (military school) in Warsaw. After graduation and nomination to the rank of Officer of the General Staff, he was sent to Łódź (October 15, 1923, to the staff of 4th Military District. At that time, he became a Freemason. On November 29, 1923, Sławek was transferred to the Officer Reserve Corps, and on June 30, 1924, he became chairman of the Association of Polish Legionnaires, together with Adam Skwarczyński. During 1926 May Coup (Poland), Sławek remained close to Piłsudski but did not take part in military activities. After the coup, he returned to active service, remaining in the army until 1928. In Polish government Soon after the May Coup, Józef Piłsudski sent Sławek to the Tarnowski family's Dzików Castle, for a series of talks with members of Polish nobility. His mission was to convince them to support the Sanacja regime. In the autumn of 1927, the informal “Council of Colonels”