Aisne-Marne, 18 July - 6 August 1918. Several days before the Germans launched their abortive Champagne-Marne drive, the French high command had made plans for a general converging offensive against the Marne salient. Petain issued orders on 12 July for the attack to begin on the 18th, with five French armies-the Tenth, Sixth, Ninth, Fifth, and Fourth, placed around the salient from left to right-taking part. Spearheading the attack were the five divisions of the French XX Corps (Tenth Army), including the American 1st and 2d Divisions. Early on 18 July the two American divisions and a French Moroccan division, jumping off behind a heavy barrage, launched the main blow at the northwest base of the salient near Soissons. Enemy frontline troops, taken by surprise, initially gave ground, although resistance stiffened after an Allied penetration of some three miles. Before the 1st and 2d Divisions were relieved (on 19 and 22 July respectively) they had advanced 6 to 7 miles, made Soissons untenable for the enemy, and captured 6,500 prisoners at a cost of over 10,000 American casualties.
Meanwhile the other French armies in the offensive also made important gains, and the German commander ordered a general retreat from the Marne salient. The French Sixth Army, on the right of the Tenth, advanced steadily from the southwest, reaching the Vesle River on 3 August. By 28 Judy this army included the American 3d, 4th, 28th, and 42d Divisions. The 4th and 42d Divisions were under control of the I Corps, the first American corps headquarters to participate in combat. On 4 August the American III Corps headquarters entered combat, taking control of the 28th and 32d Divisions (the latter had relieved the 3d Division in the line on 29 July). By 5 August the entire Sixth Army front was held by the two American corps. East of the Sixth Army the French Ninth and Fifth Armies also advanced into the salient. The Germans retired across the Aisne and Vesle Rivers, resolutely defending each strong point as they went.
By 6 August the Aisne-Marne Offensive was over. The threat to Paris was ended by wiping out the Marne salient. The initiative now had definitely passed to the Allies, ending any possibility that Ludendorff could carry out his planned offensive in Flanders. Moreover, the success of the offensive revealed the advantages of Allied unity of command and the fighting qualities of American units. The eight A.E.F. divisions (1st, 2d, 3d, 4th, 26th, 28th, 32d, 42d) in the action had spearheaded much of the advance, demonstrating offensive capabilities that helped to inspire new confidence in the war-weary Allied armies. About 270,000 Americans took part in the battle.
On 24 July, while the Aisne-Marne drive was under way, Foch had outlined his plans for the remainder of 1918 at the only conference of Allied commanders that he called during the war. He proposed that the immediate objective of the Allied offensive should be the reduction of the three main German salients (Marne, Amiens, St. Mihiel), with the goal of improving lateral communications behind the front in preparation for a general offensive in the fall. Reduction of the St. Mihiel salient was assigned to Pershing at his own request.
The excellent showing made by American troops in the Aisne-Marne Offensive gave Pershing an opportunity to press again for the formation of an independent American army. Preliminary steps in the organization of the American First Army had been taken in early July 1918. On the 4th Lt. Col. Hugh A. Drum was selected as chief of staff and directed to begin establishment of army headquarters. After conferences on 10 and 21 July, Foch agreed on the 22d to the formal organization of the First Army, and to the formation of two American sectors-a temporary combat sector in the Chateau-Thierry region, where the already active I and III Corps could comprise the nucleus of the First Army, and a quiet sector farther east, extending from Nomeny (east of the Moselle) to a point north of St. Mihiel-which would become the actual theater of operations for the American Army as soon as circumstances permitted concentration of A.E.F. divisions there. Orders issued on 24 July announced formal organization of the First Army, effective on 10 August; designated Pershing as its commander; and located its headquarters at La Ferté-sous-Jouarre, west of Chateau-Thierry.
Stabilization of the Vesle River front in early August led Pershing to alter his plane for forming the First Army. Instead of organizing it in the Chateau-Thierry region and then moving it eastward for the St. Mihiel Offensive, he secured Foch's consent on 9 August to a build-up of First Army units in the vicinity of the St. Mihiel salient. Tentative plans for reduction of the salient called for the concentration of three American corps (about 14 American and 3 French divisions) on a front extending from Port-sur-Seille westward around the bulge to Watronville. Three American divisions would remain on the Vesle front.
Meanwhile Allied forces, including American units operating in other sectors of the Western Front, were making significant gains in the preliminary phases of the great final offensives. For the sake of clarity, the role of American units in the Somme Offensive (8 August11 November), Oise-Aisne (18 August-11 November), and Ypres-Lys (19 August-11 November) Campaigns will be described briefly, before considering in more detail the activities of the main body of A.E.F. troops in the St. Mihiel (12-16 September) and Meuse-Argonne (26 September-11 November) Campaigns.
The eight A.E.F. divisions (1st, 2d, 3d, 4th, 26th, 28th, 32d, 42d) in the action had spearheaded much of the advance, demonstrating offensive capabilities that helped to inspire new confidence in the war-weary Allied armies.