Final answer:
The American Expeditionary Force (AEF) saw their first serious combat in the spring of 1918, engaging in their first major offensive at the Battle of Cantigny on May 28. Following this, they played a crucial role in subsequent victories at Chateau-Thierry and Belleau Wood, which contributed to turning the tide against the German advance.
Step-by-step explanation:
The American Expeditionary Force (AEF) saw their first serious fighting during World War I in the spring of 1918. Under the leadership of General John J. Pershing, the AEF engaged in their first major offensive at the Battle of Cantigny on May 28, 1918. This battle marked the American troops' initial significant action against German forces, and it provided a much-needed boost to the Allied forces.
Following the Battle of Cantigny, the AEF continued to play an essential role in subsequent battles, including those at Chateau-Thierry and Belleau Wood. During the battles of the summer of 1918, despite the high casualties including at Belleau Wood, the AEF helped to repel the German advance and pave the way for the Allied counteroffensives.
By the end of July 1918, the Allied Powers had taken the initiative, with the AEF contributing significantly to the success of operations at the Second Battle of the Marne and subsequent advances against the German forces, ultimately leading to the German retreat.