When the United States entered World War I, Fechet was promoted to temporary lieutenant colonel in the Signal Corps was assigned to the expanding Aviation Section on August 5, 1917 and served successively as commander of the Signal Corps Aviation School at Belleville, Illinois in 1917; commander of the Aviation School at Arcadia, Florida, in 1918; and Air Officer, Southern Department at Fort Sam Houston, Texas, 13 May 1919 – 1 Oct 1920. He received further wartime promotion to colonel in March 1918. Fechet was promoted to permanent major and lieutenant colonel, Cavalry, on 1 July 1920 when the Army Reorganization Act took effect, and formally transferred in grade to the Air Service, now a combatant arm of the line by act of Congress, on August 6, which made him 7th in seniority among officers in the branch.