In early 1939, Sima fled to Nazi Germany through Yugoslavia, wherein he and a number of Romanian exiles attempted to plan a coup in early 1939; this plot was discovered by German police and a number of Legionnaires were arrested. In the summer of the same year, he was sent back to prepare for and conduct the assassination of the Romanian Prime Minister, Armand Călinescu, on 21 September 1939 — following Călinescu's murder, a group of Legionnaires led by Sima assumed control of Romania's national radio broadcaster.