When the USA declared war in April 1917, President Woodrow Wilson sent the American Expeditionary Force (AEF) under the command of General John Pershing to the Western Front. By May 1918, there were over 500,500 US soldiers in France.
The German held St Mihiel salient was chosen for the US Army's first offensive. Pershing and 300,000 troops assembled at this sector in early September. The German High Command, aware the attack was coming, ordered a partial withdrawal of troops.
The withdrawal was still in progress when the US Army attacked on 12th September. A secondary assault, by 110,000 French troops, took place three hours later. Over 1,400 aircraft under the command of General William Mitchell, supported the advancing US and French troops. On the first day the main attack advanced 9km to reach Thiancourt and the the French troops captured the village of Dommartin. By 16th September, the entire St Mihiel salient was under Allied control.
The Western Front, July-November, 1918