The city was occupied by Germany in 1941, and all institutions of higher education for Poles were closed. From 1940 until September 1944, under Lithuanian professor and activist Mykolas Biržiška, the University of Vilnius was open for Lithuanian students under the supervision of the German occupation authorities. In 1944, many of Polish students took part in Operation Ostra Brama. The majority of them were later arrested by the NKVD and suffered repressions from their participation in the Armia Krajowa resistance.