[1] Towards the end of World War II, being close to the United States consul Burton Y. Berry, he militated for the retrocession of Northern Transylvania (ceded by Romania to Hungary in 1940 as a result of the Second Vienna Award).

[2] Between 1945 and 1947, he was the vice-president of the Romanian-American Friendship Association and gave lectures praising the United States as a bastion of democracy and praising its contributions to the defeat of the Axis powers,[3] at a time when many intellectuals joined ARLUS [ro] (the Romanian Association for Strengthening Ties with the USSR).

Following a denunciation by a colleague from the University of Cluj, he was arrested on March 26, 1948 and accused of espionage and treason, based on his alleged membership in an anti-communist resistance movement.

Mărgineanu served sixteen years (1948–1964), being incarcerated at the notorious Văcărești, Jilava, Gherla, Dej, Pitești, and Aiud prisons.

[2][4] In December 1951, together with Mircea Vulcănescu (who was detained with him at Aiud), he planned a mass escape of the prisoners, so that, once they were free, they would contact the anti-communist resistance in the mountains.