Final answer:
American soldiers saw their first serious fighting in World War I, with significant engagement beginning in 1918, contributing to the Allied victory.
Step-by-step explanation:
The first serious fighting involving American soldiers was during the Great War, known as World War I, which lasted from 1914-1918. However, the United States did not officially join the conflict until 1917. Large-scale American involvement and combat on the Western Front began in 1918. Prior to official entry, some American volunteers, like writer Ernest Hemingway, had already seen action with other armies.
The war quickly devolved into a stalemate characterized by trench warfare with high casualties due to advanced industrialized weaponry. By the war's end, the U.S. had mobilized over 4 million soldiers, with significant loss of life, although the American fatalities paled in comparison to the European toll. Despite limited combat action for many, the impact was profound, shaping subsequent American literature, art, and society.
Factors such as the introduction of American troops, military alliance with France, and the fresh resources brought by the U.S. played crucial roles in the eventual Allied victory. The brutality and sheer scale of World War I's trench warfare and the use of chemical weapons marked a devastating chapter in military history.