A French-born American citizen, Robert Soubiran felt it was his duty to go and fight for the country of his birth at the start of the First World War, travelling to France at his own expense, to join the French Foreign Legion on his arrival. He joined with fellow future Lafayette Escadrille pilots James Bach, Bert Hall, Kiffin Rockwell, Paul Rockwell and William Thaw and served with the 2nd Foreign Regiment in the Battle of Champagne, with the 170th Line Infantry Regiment. 

Injured during the fighting in October 1915, he spent four months recuperating, during which time he applied for and was accepted into the French Service Aeronautique, graduating on 20th October 1916 and joining Escadrille Américaine two days later. Around one year of combat flying later, Soubiran was awarded the Croix de Guerre with Palm for actions covering an attack against German Drachen balloons, where he was credited with his only aerial victory of the war. 

He transferred to the 103rd Pursuit Squadron US Army Air Service following America's entry into the war, where he continued to fly combat missions, eventually rising to the position of Commanding Officer. One of the first American nationals to arrive in France in 1914, Soubiran was also one of the last to leave in 1919 following the end of the war, having accumulated over 400 hours of combat flying.