The squadron, after basic flight training at Kelly Field in Curtiss JN-4 "Jenny" trainers, left for the Aviation Concentration Center at Camp Mills, Garden City, New York, on 30 September.

On the 13th, the unit boarded HMS Hunscraft at Southampton Docks and made the cross-channel trip to Le Havre, France, and at 21:30 on the 15th, arrived at the Headquarters Air Service, Zone of Advance at Chaumont Hill 402 Aerodrome.

On 30 June news was received that both airmen were prisoners, unharmed after their aircraft was attacked and disabled, being forced to land behind enemy lines.

On 25 June, Lt Cook was performing a visual reconnaissance mission and just before crossing the lines, a French Nieuport 28 was seen approaching his aircraft.

[3] During late June, the squadron was engaged in intense combat flying in support of the Battle of Château-Thierry, taking photos of practically the entire sector.

On the way back to Gondreville, four German Pfalz D.VIII scout aircraft attacked the formation, and the squadron lost one observer, Lt Baker was mortally wounded.

During the remainder of August, good flying weather kept the squadron quite busy, and by the end of the month, practically every square foot of the sector had been photographed.

The third then broke off the attack and headed towards home, however Lt Foster's aircraft was badly damaged and he went into a spiral, however he was able to land in enemy territory and was taken prisoner, unhurt.

[3] On 7 September 1918 Lts A. W. Lawson {pilot} and H. W. Verwohlt {observer} were shot down and captured near Conflans {Verwodlt was wounded in the leg, which had to be amputated on 11 November 1918}[9] On 11 September news was received that the first All-American offensive was to start the next morning with a large artillery bombardment, followed by an infantry advance to wipe out the St. Mihiel salient.

Flights were flown in darkness, searching for enemy troop concentrations and artillery batteries, returning to Gondreville by the aid of flares at night.

About 75 rounds from a pair of Lewis machine guns, sufficed and one enemy aircraft was shot down near Orly Ferme.

By early October, with the American infantry advancing on all points between the Argonne and the Meuse, German aircraft were concentrating all available resources to stop the Air Service from getting information about its rear areas.

On the 9th Lt. Kenney and his observer where attacked near Jametz in which one enemy aircraft was shot down and others driven off-Kenny would later be awarded the DSC.

Lieutenants Strahm and Jervey and Lambert and Pope started on a photographic mission between Montmedy and Longuyon when they were attacked by 14 enemy Fokkers.

[3] On the last day of combat operations, 10 November, the 91st had its final casualty when Lieutenants Bruch and White's aircraft was shot down on a mission over Stenay.

[10] The citation which was signed by French General Philippe Pétain and accompanied the award read: "The first squadron of the American Army called to execute long distance reconnaissance, it immediately realized, with admirable comprehension, the needs of high command.

[11] On 21 November, the first move was made from Vavincourt to Preutin Aerodrome, a former German airdrome, roughly 25 miles (40 km) northwest of Metz.

As a field occupied by the Germans a very short time before, Preutin was populated by a number of Fokkers and other aircraft, wrecked by American infantry on their passage through the area.

A saving feature of the stay at Preutin With the nearness of Metz and Luxembourg, both cities that all were anxious to visit, and to which parties went daily.

It was at Trier that the 91st had its first opportunity to fly German aircraft turned over to the Allies, and for taking rides in the Fokker or the Pfaltz became a popular pastime.

No one had home comforts as Christmas stockings, but the atmosphere at the dinner given in the hangar by the men at two o'clock, to which all the officers originally in the squadron were invited, was cheerful if not warm.

The menu was excellent, and the tree in the center of the hangar was decorated as they were at home, made everyone happy (or homesick, as the case may be), just to look at if.

[10] Finally in mid April, orders were received to move to the 1st Air Depot at Colombey-les-Belles Airdrome, at which all equipment would be turned in prior to departing France for the United States.

[11] Personnel at Colombey were subsequently assigned to the commanding general, services of supply, and ordered to report to the Brest, France staging camp on 19 May.