St. Mihiel, 12 - 16 September 1918. By September 1918, with both the Marne and the Amiens salients eliminated, there remained but one major threat to lateral rail communications behind the Allied lines-the old St. Mihiel salient near the Paris-Nancy line. Active preparations for its reduction began with the transfer of Headquarters First Army, effective 13 August, from La Ferté-sous-Jouarre in the Marne region to Neufchateau on the Meuse, immediately south of St. Mihiel. On 28 August the first echelon of headquarters moved closer to the front at Ligny-en-Barrois.
American unite from Flanders to Switzerland were shifted into the area near the salient. The fourteen American and four French divisions assigned to the First Army for the operation contained ample infantry and machinegun units for the attack. But because of the earlier priority given to shipment of infantry (at the insistence of the British and French) the First Army was short of artillery, tank, air and other support units essential to a well-balanced field army. The French made up this deficiency by loaning Pershing over half the artillery and nearly half the airplanes and tanks needed for the St. Mihiel operation.
Shortly before the offensive was to begin, Foch threatened once again to disrupt Pershing's long-held desire to carry out a major operation with an independent American force. On 30 August the Allied Commander in Chief proposed to exploit the recently gained successes on the Aisne-Marne and Amiens fronts by reducing the size of the St. Mihiel attack and dividing the American forces into three groups-one for the salient offensive and two for fronts to the east and west of the Argonne Forest. Pershing, however, remained adamant in his insistence that the First Army should not now be broken up, no matter where it might be sent into action. Fina1ly a compromise was reached. The St. Mihiel attack was subordinated to the much larger offensive to be launched on the Meuse-Argonne front in late September, but the First Army remained intact. Pershing agreed to limit his operations by employing only the minimum force needed to reduce the salient in three or four days. Simultaneously he was to prepare his troops for a major role in the Meuse-Argonne drive.
The St. Mihiel offensive began on 12 September with a threefold assault on the salient. The main attack was made against the south face by two American corps. On the right was the I Corps (from right to left the 82d, 90th, 5th, and 2d Divisions in line with the 78th in reserve) covering a front from Pont-à-Mousson on the Moselle westward to Limey; on the left, the IV Corps (from right to left the 89th, 42d, and 1st Divisions in line with the 3d in reserve) extending along a front from Limey westward to Marvoisin. A secondary thrust was carried out against the west face along the heights of the Meuse, from Mouilly north to Haudimont, by the V Corps (from right to left the 26th Division, the French 15th Colonial Division, and the 8th Brigade, 4th Division in line with the rest of the 4th in reserve). A holding attack against the apex, to keep the enemy in the salient, was made by the French II Colonial Corps (from right to left the French 39th Colonial Division, the French 26th Division, and the French 2d Cavalry Division in line). In First Army reserve were the American 35th, 80th, and 91st Divisions.
Tota1 Allied forces involved in the offensive numbered more than 650,000-some 550,000 American and 100,000 Allied (mostly French) troops. In support of the attack the First Army had over 3,000 guns, 400 French tanks, and 1,500 airplanes. Col. William Mitchell directed the heterogeneous air force, composed of British, French, Italian, Portuguese, and American units, in what proved to be the largest single air operation of the war. American squadrons flew 609 of the airplanes, which were mostly of French or British manufacture.
Defending the salient was German "Army Detachment C," consisting of eight divisions and a brigade in the line and about two divisions in reserve. The Germans, now desperately short of manpower, had begun a step-by-step withdrawal from the salient only the day before the offensive began. The attack went so well on 12 September that Pershing ordered a speedup in the offensive. By the morning of 13 September the 1st Division, advancing from the east, joined hands with the 26th Division, moving in from the west, and before evening all objectives in the salient had been captured. At this point Pershing halted further advances so that American units could be withdrawn for the coming offensive in the Meuse-Argonne sector.
This first major operation by an American Army under its own command took 16,000 prisoners at a cost of 7,000 casualties, eliminated the threat of an attack on the rear of Allied fortifications at Nancy and Verdun, greatly improved Allied lateral rail communications, and opened the way for a possible future offensive to seize Metz and the Briey iron fields.