Which war was fought by the United States in the 1900s?

The correct choice, which identifies a war fought by the United States in the 1900s, encompasses several significant conflicts. World War I began in 1914, and the United States entered the war in 1917, becoming a major participant in the Allied Powers' efforts against the Central Powers. This conflict marked a pivotal moment in U.S. history, shifting the nation into a global military power and significantly influencing international relations in the 20th century.

Other options reflect wars that either occurred before the 1900s or overlap into the 1900s but are not exclusively identified with that century. The Civil War took place from 1861 to 1865, while World War II spanned from 1939 to 1945, and the Korean War occurred between 1950 and 1953. Despite these wars being significant, only World War I aligns with the specified timeframe of the early 1900s, making it the correct answer in this context.