When Lithuania gained Vilnius as a result of the Soviet–Lithuanian Mutual Assistance Treaty in October 1939, Landsbergis became the chief engineer of Vilnius. He helped Balys Dvarionas organizing the Lithuanian National Symphony Orchestra in 1940 and prepared a project for a 1,200-seat opera and ballet theater (it was not constructed). After Lithuania was occupied by the Soviet Union in June 1940, Landsbergis became the chief architect of Vilnius. When Nazi Germany invaded the Soviet Union and Lithuanians started the anti-Soviet June Uprising, Landsbergis became the minister of infrastructure in the short-lived Provisional Government of Lithuania. He continued to work as chief of the city's construction board which prepared Vilnius' general plan in January 1944. During his tenure, the city began construction of a road to Kaunas as well as reconstruction of the Cathedral Square and the present-day Daukanto Square. These were not new initiaves, but a continuation of pre-war Polish projects. In 1942–1943, he also taught at Vilnius University.