In three days of brilliant offensive action, 12-14 September 1918, the American First Army attacked the salient at St-Mihiel which the Germans had held since September 1914. The salient covered the most sensitive section of the enemy's position on the western front (Mezieres-Sedan-Metz railroad and the Briey Iron Basin). It threatened the entire region between Verdun and Nancy and interrupted the main railroad line from Paris to the east. Its primary strength lay in the natural defensive features of the terrain itself. The western face of the salient extended along the rugged, heavily wooded heights of the Meuse; the southern face followed the height of the Meuse for five miles to the east and then crossed the plain of the Woevre, including with the German lines the detached heights of Loupmont and Mont Sec which dominated the plain and afforded the enemy unusual facilities for observation. The enemy had reinforced the positions by every artificial means during a period of four years.
Having concentrated by night movements over 600,000 men on the battlefield, the American troops were deployed in attack positions on the night of September 11th. On the south face of the salient was I Corps with four divisions in line extending from the Moselle westward. On its left was IV Corps with three divisions in line facing Mont Sec. These two corps delivered the main attack, the advance pivoting on the center of I Corps with the VI Corps (3 divisions) on the west. In spite of the determined resistance of the Germans who repeatedly launched strong counter-attacks, the Americans continued to progress along the whole line. The swiftness with which the operation was carried out enabled the First Army to smother the opposition to such an extent that 16,000 prisoners; 443 field guns, and large stores of material and supplies were captured.
The weight of American arms was now shifted to the Meuse-Argonne offensive which began on 26 September and ended on 11 November 1918. The transfer of American units to this area was begun even before the completion of the St-Mihiel area. The First Army under command of Lieutenant General Bullard was to hold in the St-Mihiel area, while the First Army under command of Lieutenant General Liggett took the offensive in the Meuse-Argonne.
The Meuse Heights and the broken hills of the Argonne Forest had been organized into almost impregnable positions by the addition of machine guns, artillery, trenches and obstacles of all kinds. Between these two natural bulwarks, lay the dominating hill of Montfaucon which afforded the Germans perfect observation, and whose inherent strength had been greatly increased by the elaborate use of field fortifications of all kinds.
The movement of men and material was made entirely under the cover of darkness, all activity suspended and the men kept in concealment during daylight hours. French soldiers remained in outpost positions until the last minute to prevent the Germans from seeing or otherwise securing information of the presence of large numbers of American soldiers in the region and thus receiving advance warning of the impending offensive.
Finally, on the night 25-26 September, the First Army stood on its new battle front ready for the momentous battle that was to begin the next day. I Corps was on the left, V Corps in the center and III Corps on the right. The artillery preparation for the attack began in full force at 2030 hours with 2700 guns. At 0530, the infantry began the assault, protected by a rolling barrage. The dense fog during the morning coupled with the difficult terrain impeded the attack, but except in front of Montfaucon, the progress on the first day was considered entirely satisfactory. III Corps drove forward vigorously to the east of Montfaucon and by early afternoon, its left flank was a mile beyond that hill. During the advance, its right flank wheeled toward the Meuse and took up a defensive position along the bluffs of the river. I Corps on the left, made a deep penetration along the Aire River while its left flank fought its way forward about one mile in the Argonne forest. On the evening of the 26th, the strong German first position was in the hands of the Americans, and Montfaucon, in the second position, had held out, but the deep salients driven into the German lines on both sides of that hill made its capture a question of merely a few hours.
From this point on, the American Army bolstered by one French Corps, had its feet firmly planted on the ground and maintained the initiative of attack. Incessant Allied offensives in process all along the western front had completely shaken the enemy defenses everywhere, and the signing of the Armistice on 11 November 1918, found the Allied Armies in hot pursuit of the retiring Germans along the whole front.