Final answer:
The American Expeditionary Force was ready and fully engaged in battle by May 1918, under the leadership of General John J. Pershing, although training schedules were accelerated due to German offensives.
Step-by-step explanation:
The American Expeditionary Force (AEF), under the command of General John J. Pershing, was prepared for battle and fully engaged by May 1918. Although General Pershing had initially hoped for a year of training for his troops, the necessity to counter the German Spring Offensive and the subsequent transfer of German forces to the Western Front after the Russian exit from the war forced an acceleration of this timeline. By spring, the AEF contributed to several key battles, including Cantigny, Chateau-Thierry, and Belleau Wood. By August, they played a significant role in the Second Battle of Marne and launched offensives in September at Saint-Mihiel and the Meuse-Argonne region. The influx of American troops was instrumental in stemming the German advance and ultimately in the success of the Allied forces in World War I.