On 2 August 1939 he was mobilized in the Polish Army as a reserve officer. He took part in the Polish Defensive War at the onset of World War II. After the Soviet invasion of Poland, his unit attempted to reach the Hungarian or Romanian borders in order to evade being captured and to find its way to France, where the Polish Army was being re-created. However, after the battle of Krasnobród on 23 September, he was taken prisoner of war by the Soviets. Through the transfer camp in Putyvl he was interned in the NKVD camp in Kozelsk, together with several thousand other Polish officers, professors, border guards and policemen. Interrogated by kombrig Vasili Mikhaylovich Zarubin, Swianiewicz spoke fluent Russian and was apparently found useful. After the start of the Katyn Massacre in the spring of 1940, he was attached to a group of c. 100 Polish officers being moved by train to a small station in Gniezdovo near Katyn. There all of his comrades were massed in buses with blindfolded windows and transported to the mass murder site, while Swianiewicz himself was withdrawn from the transport.