When the Communist Party of Denmark got an offer from the Comintern in 1925 to send a party member to Moscow to attend the new Lenin courses, Larsen was chosen to go. The courses were created to educate loyal leaders to the international branches of the Comintern and was planned to last for eight months. The courses were in German, English, Russian or French so the student the party was to send to Moscow had to have good language skills. His secondary education gave Larsen a head start, and in September 1925 he left Denmark for Moscow.