The correct answer is the Spanish American War.
The Spanish American War took place in 1898, right before the beginning of the 20th century.
The Mexican American War took place in 1848, essentially in the middle of the 19th century.
The US Civil War took place from 1860-1865, again this would be considered moreso the middle of the 19th century.
Last but not least, World War I does not begin until 1914, which is considered the early part of the 20th century.
