On 12th September, 1918, the American Expeditionary Force, commanded by General Pershing, lanuched the Saint-Mihiel Offensive, it's first independent action in Europe against Germany during World War One. Around 300,000 American troops supported by over 100,000 French troops were involved.
The offensive was intended to reduce the Saint-Mihiel salient, a bulge in the Western Front located to the west of Metz between Verdun and Nancy. The salient interuppted communications along the allied line and needed to be removed before a general push by the Allies against the German army could take place along the whole front.
At 5am on the morning of the 12th September German forces were subjected to an artillery barrage followed by attack by infantry supported by tanks. In anticipation of the attack, German troops had been pulling out of the salient for several days and the Americans met with light resistance. By 16th September the Americans had cleared the salient of German troops.
Following the successful conclusion of the offensive, the AEF moved north to take part in the Meuse-Argonne Offensive.