The battle of St. Mihiel saw the first major independent American offensive of the First World War. General John Pershing had generally resisted British and French attempts to feed American troops into the frontline as soon as they were available, instead preferring to concentrate his troops in a single army. On 30 August 1918 the First American Army was finally ready to enter the battle. The new army was immediately deployed to the south side of the St. Mihiel salient. This salient, south of Verdun, had been in German hands since 1914, but in the autumn of 1918, the Germans were in retreat. In mid-September they were about to abandon the St. Mihiel salient to withdraw to the Hindenburg Line.
The German retreat began on 11 September. The next morning the Americans attacked. Pershing committed two corps (I and IV) to the attack. The attack was backed up by an artillery barrage from 2,900 guns (many French) as well as a force of French tanks and a French Colonial division.
The Germans were caught by surprise. Outnumbered and slightly out of position, the German position collapsed. In 36 hours the Americans took over 13,000 prisoners and captured 466 guns. The Germans lost 5,000 killed and wounded, while the Americans suffered 7,000 casualties.
What the American troops lacked in experience of the Western Front they made up for in enthusiasm and morale. The first American troops had arrived in France in the summer of 1917 and had been waiting to enter the fray ever since. In contrast the arrival of increasingly large numbers of American soldiers on the front line had a very damaging impact on German morale.
With the salient eliminated, Marshal Foch ended the American offensive. The Americans were moved to the Argonne Forest, where they would make their main contribution to the vital fighting on the Western Front.