[1] He then enrolled in the Faculty of Sciences at the University of Iași, but his studies were interrupted by the entry of Romania in World War I; conscripted into the army, he fought as a second lieutenant at the battles of Nămoloasa, Băltăreți, and Mărășești in the summer of 1917.

[1][2] General Henri Mathias Berthelot, the head of the French military mission to Romania, decided to send a group of young Romanians (including Hulubei) to France to train at an aviation school; upon completing the training, Hulubei participated as a pilot on a fighter aircraft of the French Air Service on the Western Front.

[1][2] Upon returning to Romania, he worked for a while in civil aviation, and helped start the first Romanian air service, connecting Istanbul to Bucharest to Budapest.

[3] His Ph.D. thesis with the title "Contribution to the study of quantum diffusion of X-rays" was defended in 1933 in Paris in front of an examination committee chaired by Nobel laureate Marie Curie.

[4] Afterwards he continued his research at the University of Paris, staying in contact with the likes of Frédéric Joliot-Curie, Paul Langevin, and Albert Einstein.