Which of the following conflicts was fought in the 20th century?

World War I is indeed a significant conflict that was fought in the 20th century, specifically between 1914 and 1918. This war involved many of the world's great powers and was marked by trench warfare and significant loss of life. It dramatically changed the political landscape of the world, leading to the collapse of empires and setting the stage for future global conflicts.

The other conflicts listed occurred in earlier centuries. The Civil War took place in the United States from 1861 to 1865, emphasizing issues like slavery and states' rights. The Mexican-American War occurred between 1846 and 1848, primarily involving territorial expansion in North America. The War of 1812, fought between the United States and Great Britain from 1812 to 1815, addressed maritime issues and national sovereignty. Each of these wars is significant in its own right but does not belong to the 20th century timeline.