After Russia conquered eastern Galicia at the start of World War I in 1914, Fahn and many other Galician Jews fled to Vienna, the capital of Austria-Hungary. Fahn enjoyed the libraries in Vienna and worked as a librarian during his time there. He was drafted into the Austro-Hungarian army on 1 December 1914. His military service ended on 2 November 1918, with Austria-Hungary's existence ending soon afterwards. Due to his house and property in Halicz being destroyed due to World War I, Fahn moved to Stanisławów (present-day Ivano-Frankivsk). Afterwards, Fahn continued his work, but as a Polish citizen.