An anatomist by specialty, Popa worked on popularizing modern science, reforming the medical and higher education systems, and, in war hospitals, as a decorated and publicly acclaimed practitioner.

He criticized Marxism as much as scientific racism, but condemned Romania's participation in the war against the Soviets, and, in 1944, joined a protest movement of high-profile academics.

His mother noticed his aptitude early on, and despite great material difficulties, including selling off their land so he could finish high school, his parents managed to provide him with an education.

[1] As argued by historian Lucian Boia, Popa's lowly origin and his successful career stand as evidence of an "upward social mobility" in the pre-1944 Kingdom of Romania.

[8] During World War I, Popa cared for the wounded and sick at Iași's Sfântul Spiridon Hospital, earning him a knighthood in the Order of the Crown.

Leading the Brotherhood's student center, he spoke in public about the United Principalities' 60th anniversary, expressing his sadness that this had not been celebrated as a national holiday in Iași.

[19] Popa identified Iași with extreme nationalism, and, in a 1925 article for the student review Viața Universitară, accused the far-right National-Christian Defense League of hypocrisy.

He had a direct experience of America, and of what he liked to call its "guided democracy",[22] which was rare among Romanians of his generation, and which he recorded in detail in diaries he intended for publishing.

[16] He spent the first year in Chicago, the second at the Marine Biological Laboratory in Woods Hole and finishing by studying Anatomy and Embryology in 1927–1928 at University College Hospital Medical School, under Grafton Elliot Smith.

His scientific activity, after his work on the dura mater, focused on three areas: the hypothalamic–pituitary–adrenal axis, the reform of medical education at the university level,[23] and the physiology of spontaneous movement (motility) in spermatozoa.

For many years he taught histology, Anatomical Pathology and Legal Medicine, as required,[27] and was also curator of Sfântul Spiridon Hospital, as well as head of the Physicians and Naturalists' Society.

[33] In January 1936, together with writers Mihail Sadoveanu, George Topîrceanu and Mihai Codreanu, Popa founded Însemnări Ieșene ("Recordings from Iași"), a magazine of commentary.

"[22] At Însemnări Ieșene, which, under Topîrceanu's guidance, made efforts to preserve its political independence,[35] Popa took a firm stand against the violently fascist Iron Guard, and denounced scientific racism.

[35] Popa criticized the breakdown of Romanian democracy and the creation of a National Renaissance Front (FRN) dictatorship in 1938, describing it as "unprecedented lunacy or the actual perversion of leadership".

[22] That year, in an obituary piece for the socialist physician Ioan Cantacuzino, Popa outlined his own humanist vision of science as a "sacred fire".

[43] In October 1939—shortly after the Invasion of Poland and the start of World War II—, Însemnări Ieșene published his article deploring man's return to his "beastly" nature and expressing fears that modern life had made soldiers indifferent about transcendentals.

[46] Harassed by the Iron Guard, which blacklisted him for assassination,[3][47] Popa managed to survive its "National Legionary State" regime, proclaimed in September 1940.

Science as a Basis for Bettering Mankind") objected to Romania's economic dependency, claiming that Romanians were at risk of falling back among "agricultural peoples", those "destined to perpetual ignorance".

In front of communist-run purging committees, he defended on professional grounds those colleagues accused of having sided with fascism, and called for the reinstatement of academic freedom.

[63] At a conference in 1945, he praised the British and American university systems, drawing a vehement letter of rebuke from Constantin Ion Parhon, who considered the Soviet model as optimal.

[64] As noted in 2009 by historian Bogdan Cristian Iacob, Popa's stance showed "a glaring lack of sense for the times", "an incapacity to grasp that the Academy and University were not, at least initially, attacked on the basis of the scholarship produced, but from political positions."

Popa signed his name to a public protest decrying vote-rigging during the November 1946 election; there were ten other signatories, including aviator Smaranda Brăescu and Army General Aurel Aldea.

From an unassuming title, which implied a lecture about "nervous tension and the century's disease",[3][40] it turned abruptly to political critique, likening the abuses of Nazism to those of communism.

"[73] His was also an appeal against immoral but "exact" science, describing ideologues as "disciples of the Antichrist": "In this grave situation, the time has come for any conscience that is still pure to ask themselves: 'Where to?'

[3] More optimistically, Constantin Rădulescu-Motru, a fellow anticommunist academician, wrote that those making an "ostentatious" exit were friends of the Soviet regime, such as Sadoveanu and Parhon.

"[75] As argued by Stanomir, Popa "spoke out inadmissible truths and gave value appraisals to a regime that was just getting ready to impose Stalinist orthodoxy upon the intellectuals.

Another means used to target him was a proposal to admit hundreds of students who had been victims of Nazi oppression in Northern Transylvania, some of whom spoke no Romanian, and award them doctorates in two or three years.

[3] By constantly changing addresses and not venturing out into the street, Popa managed to evade arrest and was finally brought home, moribund, at the beginning of July 1948.

[17] Over the course of the communist period, there was a concerted effort to banish Popa's memory,[3] even though his 1944 petition to Antonescu was still being officially cited as evidence of a communist-backed resistance movement to fascism.

[16][34] This is one of several monographs and anthologies edited by Constantinescu, detailing such topics as Popa's Christian faith and his correspondence with poet-physician Vintilă Ciocâlteu, and including his American diary (published 2014).