My guess would be the Air Corp ?

Marines

US ARMY AIR SERVICE

1) According to various trivia sources, this should have been the US Air Force. For instance here: "- What current branch of the U.S. Military was a corp of only 50 Soldiers when World War 1 broke out? - United States Airforce - Air Force is correct!!!" Source and further information: http://groups.msn.com/TheScottFamilyandFriends/dailytrivia.msnw?action=get_message&mview=0&ID_Message=8871&LastModified=4675679423860316711 "The Answer to March’s Trivia Question is: The U.S. Air Force is the current branch in the U.S. Military that was a corps of only 50 soldiers when World War 1 broke out." Source and further information: http://txins.com/newsletters/2007-4Newsletter.pdf 2) Note that WW1 broke out in 1914, but the United States' entry into World War I was in 1918, at the end of the war. 3) "The Aeronautical Division, U.S. Signal Corps (1907-1914) was the first progenitor of the United States Air Force, and as such is the first military air organization. A component of the U.S. Army Signal Corps, the Aeronautical Division did not contain any subordinate units during its existence." "The Army Air Forces Statistical Digest (World War II) (Table 3, "AAF Military personnel--number and percent of US Army strength") listed the strength of the division at 51 officers and men on November 1, 1912, and 114 on September 30, 1913. In the following year Congress increased the size and prestige of Signal Corps aviation by enacting a law established an Aviation Section, U.S. Signal Corps to replace the Aeronautical Division on July 18, 1914." Source and further information: http://en.wikipedia.org/wiki/Aeronautical_Division,_U.S._Signal_Corps 4) "The Aviation Section, U.S. Signal Corps, was the name of the military aviation service of the United States Army from 1914 to 1918, and a direct ancestor of the United States Air Force. It replaced the Aeronautical Division, U.S. Signal Corps and was succeeded briefly by the Division of Military Aeronautics, Secretary of War, and then the U.S. Army Air Service. "The Aviation Section, U.S. Signal Corps was created by the 63rd Congress (38 Stat. 514) on July 18, 1914, to replace the Aeronautical Division after earlier legislation to make the aviation service independent from the Signal Corps died in committee. The new law authorized a significant increase in size of U.S. military aviation to 60 officers and 260 enlisted men, but stipulated that most be volunteers from other branches of the Army than the Signal Corps, which by regulation limited their time of service away from their regular units to four years. The first funding appropriation for the Aviation Section was $250,000 for fiscal year 1915. It also decreed restrictions that officers detached to the section be unmarried and no higher in rank than 1st lieutenant, both of which led to a lack of discipline and professional maturity among the aviators that severely handicapped the growth of the service. At its creation, the Aviation Section had 19 officers and 101 enlisted men. On 5 August, 1914, it was organized into four subordinate organizations: the Signal Corps Aviation School, the 1st Aero Squadron, and the 1st and 2nd Companies of the squadron, totalling 16 officers, 92 enlisted men, seven civilians, and seven aircraft." Source and further information: http://en.wikipedia.org/wiki/Aviation_Section,_U.S._Signal_Corps According to those references, the size of 50 soldiers seems to apply to the Aeronautical Division, U.S. Signal Corps in 1912. It was certainly not a "corps" at that time.