<p>The US campaign at St-Mihiel, which followed, was planned as a drive to pinch off the German salient on the Meuse River south of Merdun. The support air force was placed again under Mitchell's command. He assembled 101 squadrons totalling 1,476 aircraft assigned. More than 700 of them were from the French Division Aérienne, a highly mobile and flexible tactical air force. About 600 were drawn from US squadrons, perhaps 130 from the British, and the rest from Italian units.
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<p>When the offensive began on 12 September 1918, Mitchell's force actually numbered about 959 planes available to fly. The weather was bad, so that most missions on the opening day were limited to single-plane reconnaissance flights. One was by Mitchell himself, who flew daily reconnaissance flights during the campaign. There was moderate fighting after the weather cleared, with the major losses suffered by the newer and inexperienced units, as usual. There was not that much pursuit action, and both their losses and victories were low.
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<p>One army and four corps observations squadrons flew the bulk of the St-Mihiel missions on reconnaissance and artillery regulation flights. The 91st Army Obs. Sqn, the only experienced and well-equipped unit in its group, flew penetration missions as deep as 50 miles (80 km) behind the German front, and at altitudes below 3,000 ft (914 m), in carrying out its mission.
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<p>The 91st operated French Salmson 2A.2 aircraft, and had gradually evolved their observation missions to meet the conditions of the Western Front. Before St-Mihiel, most of their missions had been flown at dawn and at an altitude of 16,000 ft (4,877 m). At St-Mihiel they shifted to low-altitude flights, and were successful. The 91st was part of the First Army Observation Group, along with the 24th and 9th Squadrons, which had been formed on 6 September during the build-up for the St-Mihiel campaign.
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<p>The 9th was a unique outfit, being the only night observation unit in the A.E.F. It was equipped with the ubiquitous Breguet 14, but it was the Corps d'Armée model, known as the 14A for short. The planes were painted black and some had rather elaborate decorations, the forerunners of shark's mouths and other wild animal and fish adaptations to aircraft noses.
