Stahl was drafted into the Romanian Land Forces in 1916, when Romania entered World War I as an enemy of Austria-Hungary. He was a reserve lieutenant in the 6th (Mihai Viteazu) Regiment. He saw action during the early stages of the Romanian Campaign, but was badly wounded in the trenches during the battles of September 1916. Losing consciousness, he was dragged away and rescued by his peasant batman, and was under treatment at Gerota hospital when Iorga dedicated him an encomium in "Neamul Românesc" (September 23, 1916). He was a convalescent in Ploiești when the Central Powers occupied southern Romania.