The Second World War started in Yugoslavia in 1941. Popescu was at the time attending the Commercial Academy in Belgrade, and by 6 April 1941, he was a second lieutenant defending the country against German forces.[1] It took four days, on 10 April, when he was captured by Axis forces near Belgrade and taken to Romania to a concentration camp in Turnu Măgurele.[1] After spending two years in the camp, in 1943 he caught the attention of Juventus Bucharest boss Cezar Popescu who got the news that this 27-year-old defender who had played in Serbia was in the camp 8.[1] By explaining how Virgil Popescu was a Romanian and as such a German ally, he managed to release him from the camp and brought him to the team.[1] He made his debut for Bucharest side Juventus on 6 October, in a match against Craiova. He made 7 appearances for Juventus in the 1943–44 Romanian Divizia A.[7] However, not very long afterwards he entered the club offices and said that he had to leave to fight alongside Yugoslav Partisans and Marshal Tito in freeing Yugoslavia, and club officials accepted his will, so he returned to Yugoslavia and joined the resistance.[1]
He fought the Germans, and at the end of the war, in 1945, he was among the founders of Belgrade-based FK Partizan which became one of the major powers of Yugoslav football.[8] His passion for Serbia was such that he adopted a Serbian name, Stanislav.[1] He played with Partizan in the Yugoslav First League for two seasons. He played a total of 65 matches and scored once for Partizan, of which 17 matches were in the league.[9] Earlier, he played with SAP Vojvodina in the 1945 Yugoslav Football Tournament.[10] With Partizan he won the first Yugoslav post-World War II championship.