 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.In May 1918, following Romania's withdrawal from war, Popa applied to rejoin Rainer's team in Bucharest, where Rainer was performing experimental surgery on wounded soldiers.In July of that year, he married Florica Cernătescu, a university classmate.