In August 1914, Christiansen was called up and posted to Zeebrugge as naval aviator. He flew Hansa-Brandenburg W.12 seaplanes over the North Sea, the English Channel, and Britain. For his bombing missions on Dover and Ramsgate, he was awarded the Iron Cross 2nd Class. From 1915 to 1916 Christiansen went on numerous reconnaissance and bombing missions, helping to make his unit at Zeebrugge one of the most successful in the German Naval Air Service. On 27 April 1916, as "Leutnant der Matrosen Artillerie" (Lieutenant of Naval Artillery,) Christiansen was awarded the Iron Cross 1st Class and Knights Cross with Swords of the House Order of Hohenzollern.