Name one war fought by the United States in the 1900s.

The choice of World War I or World War II is correct because both of these significant conflicts were indeed fought by the United States during the 1900s. World War I lasted from 1914 to 1918, with the U.S. joining the Allies in 1917. World War II spanned from 1939 to 1945, with U.S. involvement beginning following the attack on Pearl Harbor in 1941. Both wars are pivotal events in American history, shaping the nation's role on the global stage and leading to significant political and social changes domestically.

The other options, while mentioning important conflicts, do not fit within the stipulated timeframe of the 1900s. The Civil War occurred from 1861 to 1865, the War of 1812 took place from 1812 to 1815, and the Spanish-American War was fought in 1898. Therefore, recognizing the importance of World War I and World War II reflects an understanding of U.S. military history specifically in the 20th century.