After the Soviet Union occupation of Lithuania in June 1940, Prapuolenis joined the Lithuanian Activist Front (LAF) and started organizing the anti-Soviet June Uprising. When Germany invaded the Soviet Union and LAF members started the uprising, Prapuolenis read the independence proclamation at 9:28 a.m. on 23 June 1941 at the Kaunas radio station. Prapuolenis became LAF representative to the Provisional Government of Lithuania. But when Prapuolenis protested the German decision to disband the Provisional Government, he was arrested by the Gestapo and sent to the Dachau concentration camp. After five months, he was freed due to the efforts of Kazys Škirpa. Prapuolenis lived in Munich under police supervision, but kept contacts with Lithuanian resistance groups. After the war, he briefly lived in Austria and Switzerland before moving to West Germany in 1948 and United States in 1955. He worked for the (BALF), was an active member of the Supreme Committee for the Liberation of Lithuania (VLIK), and board member of the Society of Friends of the Lithuanian Front.
