Emsden volunteered to join the military in 1914 during World War I to serve his country, and was sent to France in 1915. As a sergeant with the Bedfordshire Regiment, and after 79 commissioned officers were killed in one day, he personally led three bayonet charges up Hill 60 at the Second Battle of Ypres, taking his objective at the third attempt. He also served with distinction at the battles of the Somme, Cambrai, and St. Quentin. Having survived those battles, he volunteered for the Royal Flying Corps.