Once Romania entered World War I on the side of the Allies in August 1916, Popescu fought in Dobruja as a battalion commander of the 40th Infantry Regiment.

After advancing in rank to lieutenant colonel in 1925, Popescu served as instructor at the Higher War School, and then as military attaché in Rome from 1928 to 1930.

He commanded the Guards Brigade from 1937 to 1939, after which he became the Deputy Chief of the General Staff and served as director of the National Military Circle, February–March 1939.

In early July he helped manage the flow of refugees from Bessarabia and Northern Bukovina in the wake of the Soviet annexation of those Romanian territories.

[3] On August 30, Popescu participated in a meeting of King Carol II's Crown Council, where he was (alongside Gheorghe Mihail, Nicolae Păiș, and Ernest Ballif [ro]) one of the four out of five military representatives who recommended accepting the provisions of the Second Vienna Award, by which Romania were to cede Northern Transylvania to Hungary.