In 1915 Moravec was drafted into Austro-Hungarian Army and sent to the Eastern Front, into Galicia. In September 1914 he fought at the Battle of Rawa. On January 13, 1915 Moravec was taken as prisoner by Russian troops and sent to the POW camp in Tsaritsyn. In 1916 he joined the Serbian Legion and fought in the Romanian Front, was moved from Archangel (Archangelsk) to Britain, and in 1917 joined the Czechoslovak Legions at the Salonica Front. In January 1918 the legions were sent to the Western Front in France and in summer 1918 to the Italian Front.